text: /*!
   * chai
   * http://chaijs.com
   * Copyright(c) 2011-2014 Jake Luer <jake@alogicalparadox.com>
   * MIT Licensed
   */
  
  module.exports = function (chai, _) {
    var Assertion = chai.Assertion
      , AssertionError = chai.AssertionError
      , flag = _.flag;
  
    /**
     * ### Language Chains
     *
     * The following are provided as chainable getters to improve the readability
     * of your assertions.
     *
     * **Chains**
     *
     * - to
     * - be
     * - been
     * - is
     * - that
     * - which
     * - and
     * - has
     * - have
     * - with
     * - at
     * - of
     * - same
     * - but
     * - does
     * - still
     *
     * @name language chains
     * @namespace BDD
     * @api public
     */
  
    [ 'to', 'be', 'been', 'is'
    , 'and', 'has', 'have', 'with'
    , 'that', 'which', 'at', 'of'
    , 'same', 'but', 'does', 'still' ].forEach(function (chain) {
      Assertion.addProperty(chain);
    });
  
    /**
     * ### .not
     *
     * Negates all assertions that follow in the chain.
     *
     *     expect(function () {}).to.not.throw();
     *     expect({a: 1}).to.not.have.property('b');
     *     expect([1, 2]).to.be.an('array').that.does.not.include(3);
     *
     * Just because you can negate any assertion with `.not` doesn't mean you
     * should. With great power comes great responsibility. It's often best to
     * assert that the one expected output was produced, rather than asserting
     * that one of countless unexpected outputs wasn't produced. See individual
     * assertions for specific guidance.
     *
     *     expect(2).to.equal(2); // Recommended
     *     expect(2).to.not.equal(1); // Not recommended
     *
     * @name not
     * @namespace BDD
     * @api public
     */
  
    Assertion.addProperty('not', function () {
      flag(this, 'negate', true);
    });
  
    /**
     * ### .deep
     *
     * Causes all `.equal`, `.include`, `.members`, `.keys`, and `.property`
     * assertions that follow in the chain to use deep equality instead of strict
     * (`===`) equality. See the `deep-eql` project page for info on the deep
     * equality algorithm: https://github.com/chaijs/deep-eql.
     *
     *     // Target object deeply (but not strictly) equals `{a: 1}`
     *     expect({a: 1}).to.deep.equal({a: 1});
     *     expect({a: 1}).to.not.equal({a: 1});
     *
     *     // Target array deeply (but not strictly) includes `{a: 1}`
     *     expect([{a: 1}]).to.deep.include({a: 1});
     *     expect([{a: 1}]).to.not.include({a: 1});
     *
     *     // Target object deeply (but not strictly) includes `x: {a: 1}`
     *     expect({x: {a: 1}}).to.deep.include({x: {a: 1}});
     *     expect({x: {a: 1}}).to.not.include({x: {a: 1}});
     *
     *     // Target array deeply (but not strictly) has member `{a: 1}`
     *     expect([{a: 1}]).to.have.deep.members([{a: 1}]);
     *     expect([{a: 1}]).to.not.have.members([{a: 1}]);
     *
     *     // Target set deeply (but not strictly) has key `{a: 1}`
     *     expect(new Set([{a: 1}])).to.have.deep.keys([{a: 1}]);
     *     expect(new Set([{a: 1}])).to.not.have.keys([{a: 1}]);
     *
     *     // Target object deeply (but not strictly) has property `x: {a: 1}`
     *     expect({x: {a: 1}}).to.have.deep.property('x', {a: 1});
     *     expect({x: {a: 1}}).to.not.have.property('x', {a: 1});
     *
     * @name deep
     * @namespace BDD
     * @api public
     */
  
    Assertion.addProperty('deep', function () {
      flag(this, 'deep', true);
    });
  
    /**
     * ### .nested
     *
     * Enables dot- and bracket-notation in all `.property` and `.include`
     * assertions that follow in the chain.
     *
     *     expect({a: {b: ['x', 'y']}}).to.have.nested.property('a.b[1]');
     *     expect({a: {b: ['x', 'y']}}).to.nested.include({'a.b[1]': 'y'});
     *
     * If `.` or `[]` are part of an actual property name, they can be escaped by
     * adding two backslashes before them.
     *
     *     expect({'.a': {'[b]': 'x'}}).to.have.nested.property('\\.a.\\[b\\]');
     *     expect({'.a': {'[b]': 'x'}}).to.nested.include({'\\.a.\\[b\\]': 'x'});
     *
     * `.nested` cannot be combined with `.own`.
     *
     * @name nested
     * @namespace BDD
     * @api public
     */
  
    Assertion.addProperty('nested', function () {
      flag(this, 'nested', true);
    });
  
    /**
     * ### .own
     *
     * Causes all `.property` and `.include` assertions that follow in the chain
     * to ignore inherited properties.
     *
     *     Object.prototype.b = 2;
     *
     *     expect({a: 1}).to.have.own.property('a');
     *     expect({a: 1}).to.have.property('b');
     *     expect({a: 1}).to.not.have.own.property('b');
     *
     *     expect({a: 1}).to.own.include({a: 1});
     *     expect({a: 1}).to.include({b: 2}).but.not.own.include({b: 2});
     *
     * `.own` cannot be combined with `.nested`.
     *
     * @name own
     * @namespace BDD
     * @api public
     */
  
    Assertion.addProperty('own', function () {
      flag(this, 'own', true);
    });
  
    /**
     * ### .ordered
     *
     * Causes all `.members` assertions that follow in the chain to require that
     * members be in the same order.
     *
     *     expect([1, 2]).to.have.ordered.members([1, 2])
     *       .but.not.have.ordered.members([2, 1]);
     *
     * When `.include` and `.ordered` are combined, the ordering begins at the
     * start of both arrays.
     *
     *     expect([1, 2, 3]).to.include.ordered.members([1, 2])
     *       .but.not.include.ordered.members([2, 3]);
     *
     * @name ordered
     * @namespace BDD
     * @api public
     */
  
    Assertion.addProperty('ordered', function () {
      flag(this, 'ordered', true);
    });
  
    /**
     * ### .any
     *
     * Causes all `.keys` assertions that follow in the chain to only require that
     * the target have at least one of the given keys. This is the opposite of
     * `.all`, which requires that the target have all of the given keys.
     *
     *     expect({a: 1, b: 2}).to.not.have.any.keys('c', 'd');
     *
     * See the `.keys` doc for guidance on when to use `.any` or `.all`.
     *
     * @name any
     * @namespace BDD
     * @api public
     */
  
    Assertion.addProperty('any', function () {
      flag(this, 'any', true);
      flag(this, 'all', false);
    });
  
    /**
     * ### .all
     *
     * Causes all `.keys` assertions that follow in the chain to require that the
     * target have all of the given keys. This is the opposite of `.any`, which
     * only requires that the target have at least one of the given keys.
     *
     *     expect({a: 1, b: 2}).to.have.all.keys('a', 'b');
     *
     * Note that `.all` is used by default when neither `.all` nor `.any` are
     * added earlier in the chain. However, it's often best to add `.all` anyway
     * because it improves readability.
     *
     * See the `.keys` doc for guidance on when to use `.any` or `.all`.
     *
     * @name all
     * @namespace BDD
     * @api public
     */
  
    Assertion.addProperty('all', function () {
      flag(this, 'all', true);
      flag(this, 'any', false);
    });
  
    /**
     * ### .a(type[, msg])
     *
     * Asserts that the target's type is equal to the given string `type`. Types
     * are case insensitive. See the `type-detect` project page for info on the
     * type detection algorithm: https://github.com/chaijs/type-detect.
     *
     *     expect('foo').to.be.a('string');
     *     expect({a: 1}).to.be.an('object');
     *     expect(null).to.be.a('null');
     *     expect(undefined).to.be.an('undefined');
     *     expect(new Error).to.be.an('error');
     *     expect(Promise.resolve()).to.be.a('promise');
     *     expect(new Float32Array).to.be.a('float32array');
     *     expect(Symbol()).to.be.a('symbol');
     *
     * `.a` supports objects that have a custom type set via `Symbol.toStringTag`.
     *
     *     var myObj = {
     *       [Symbol.toStringTag]: 'myCustomType'
     *     };
     *
     *     expect(myObj).to.be.a('myCustomType').but.not.an('object');
     *
     * It's often best to use `.a` to check a target's type before making more
     * assertions on the same target. That way, you avoid unexpected behavior from
     * any assertion that does different things based on the target's type.
     *
     *     expect([1, 2, 3]).to.be.an('array').that.includes(2);
     *     expect([]).to.be.an('array').that.is.empty;
     *
     * Add `.not` earlier in the chain to negate `.a`. However, it's often best to
     * assert that the target is the expected type, rather than asserting that it
     * isn't one of many unexpected types.
     *
     *     expect('foo').to.be.a('string'); // Recommended
     *     expect('foo').to.not.be.an('array'); // Not recommended
     *
     * `.a` accepts an optional `msg` argument which is a custom error message to
     * show when the assertion fails. The message can also be given as the second
     * argument to `expect`.
     *
     *     expect(1).to.be.a('string', 'nooo why fail??');
     *     expect(1, 'nooo why fail??').to.be.a('string');
     *
     * `.a` can also be used as a language chain to improve the readability of
     * your assertions.
     *
     *     expect({b: 2}).to.have.a.property('b');
     *
     * The alias `.an` can be used interchangeably with `.a`.
     *
     * @name a
     * @alias an
     * @param {String} type
     * @param {String} msg _optional_
     * @namespace BDD
     * @api public
     */
  
    function an (type, msg) {
      if (msg) flag(this, 'message', msg);
      type = type.toLowerCase();
      var obj = flag(this, 'object')
        , article = ~[ 'a', 'e', 'i', 'o', 'u' ].indexOf(type.charAt(0)) ? 'an ' : 'a ';
  
      this.assert(
          type === _.type(obj).toLowerCase()
        , 'expected #{this} to be ' + article + type
        , 'expected #{this} not to be ' + article + type
      );
    }
  
    Assertion.addChainableMethod('an', an);
    Assertion.addChainableMethod('a', an);
  
    /**
     * ### .include(val[, msg])
     *
     * When the target is a string, `.include` asserts that the given string `val`
     * is a substring of the target.
     *
     *     expect('foobar').to.include('foo');
     *
     * When the target is an array, `.include` asserts that the given `val` is a
     * member of the target.
     *
     *     expect([1, 2, 3]).to.include(2);
     *
     * When the target is an object, `.include` asserts that the given object
     * `val`'s properties are a subset of the target's properties.
     *
     *     expect({a: 1, b: 2, c: 3}).to.include({a: 1, b: 2});
     *
     * When the target is a Set or WeakSet, `.include` asserts that the given `val` is a
     * member of the target. SameValueZero equality algorithm is used.
     *
     *     expect(new Set([1, 2])).to.include(2);
     *
     * When the target is a Map, `.include` asserts that the given `val` is one of
     * the values of the target. SameValueZero equality algorithm is used.
     *
     *     expect(new Map([['a', 1], ['b', 2]])).to.include(2);
     *
     * Because `.include` does different things based on the target's type, it's
     * important to check the target's type before using `.include`. See the `.a`
     * doc for info on testing a target's type.
     *
     *     expect([1, 2, 3]).to.be.an('array').that.includes(2);
     *
     * By default, strict (`===`) equality is used to compare array members and
     * object properties. Add `.deep` earlier in the chain to use deep equality
     * instead (WeakSet targets are not supported). See the `deep-eql` project
     * page for info on the deep equality algorithm: https://github.com/chaijs/deep-eql.
     *
     *     // Target array deeply (but not strictly) includes `{a: 1}`
     *     expect([{a: 1}]).to.deep.include({a: 1});
     *     expect([{a: 1}]).to.not.include({a: 1});
     *
     *     // Target object deeply (but not strictly) includes `x: {a: 1}`
     *     expect({x: {a: 1}}).to.deep.include({x: {a: 1}});
     *     expect({x: {a: 1}}).to.not.include({x: {a: 1}});
     *
     * By default, all of the target's properties are searched when working with
     * objects. This includes properties that are inherited and/or non-enumerable.
     * Add `.own` earlier in the chain to exclude the target's inherited
     * properties from the search.
     *
     *     Object.prototype.b = 2;
     *
     *     expect({a: 1}).to.own.include({a: 1});
     *     expect({a: 1}).to.include({b: 2}).but.not.own.include({b: 2});
     *
     * Note that a target object is always only searched for `val`'s own
     * enumerable properties.
     *
     * `.deep` and `.own` can be combined.
     *
     *     expect({a: {b: 2}}).to.deep.own.include({a: {b: 2}});
     *
     * Add `.nested` earlier in the chain to enable dot- and bracket-notation when
     * referencing nested properties.
     *
     *     expect({a: {b: ['x', 'y']}}).to.nested.include({'a.b[1]': 'y'});
     *
     * If `.` or `[]` are part of an actual property name, they can be escaped by
     * adding two backslashes before them.
     *
     *     expect({'.a': {'[b]': 2}}).to.nested.include({'\\.a.\\[b\\]': 2});
     *
     * `.deep` and `.nested` can be combined.
     *
     *     expect({a: {b: [{c: 3}]}}).to.deep.nested.include({'a.b[0]': {c: 3}});
     *
     * `.own` and `.nested` cannot be combined.
     *
     * Add `.not` earlier in the chain to negate `.include`.
     *
     *     expect('foobar').to.not.include('taco');
     *     expect([1, 2, 3]).to.not.include(4);
     *
     * However, it's dangerous to negate `.include` when the target is an object.
     * The problem is that it creates uncertain expectations by asserting that the
     * target object doesn't have all of `val`'s key/value pairs but may or may
     * not have some of them. It's often best to identify the exact output that's
     * expected, and then write an assertion that only accepts that exact output.
     *
     * When the target object isn't even expected to have `val`'s keys, it's
     * often best to assert exactly that.
     *
     *     expect({c: 3}).to.not.have.any.keys('a', 'b'); // Recommended
     *     expect({c: 3}).to.not.include({a: 1, b: 2}); // Not recommended
     *
     * When the target object is expected to have `val`'s keys, it's often best to
     * assert that each of the properties has its expected value, rather than
     * asserting that each property doesn't have one of many unexpected values.
     *
     *     expect({a: 3, b: 4}).to.include({a: 3, b: 4}); // Recommended
     *     expect({a: 3, b: 4}).to.not.include({a: 1, b: 2}); // Not recommended
     *
     * `.include` accepts an optional `msg` argument which is a custom error
     * message to show when the assertion fails. The message can also be given as
     * the second argument to `expect`.
     *
     *     expect([1, 2, 3]).to.include(4, 'nooo why fail??');
     *     expect([1, 2, 3], 'nooo why fail??').to.include(4);
     *
     * `.include` can also be used as a language chain, causing all `.members` and
     * `.keys` assertions that follow in the chain to require the target to be a
     * superset of the expected set, rather than an identical set. Note that
     * `.members` ignores duplicates in the subset when `.include` is added.
     *
     *     // Target object's keys are a superset of ['a', 'b'] but not identical
     *     expect({a: 1, b: 2, c: 3}).to.include.all.keys('a', 'b');
     *     expect({a: 1, b: 2, c: 3}).to.not.have.all.keys('a', 'b');
     *
     *     // Target array is a superset of [1, 2] but not identical
     *     expect([1, 2, 3]).to.include.members([1, 2]);
     *     expect([1, 2, 3]).to.not.have.members([1, 2]);
     *
     *     // Duplicates in the subset are ignored
     *     expect([1, 2, 3]).to.include.members([1, 2, 2, 2]);
     *
     * Note that adding `.any` earlier in the chain causes the `.keys` assertion
     * to ignore `.include`.
     *
     *     // Both assertions are identical
     *     expect({a: 1}).to.include.any.keys('a', 'b');
     *     expect({a: 1}).to.have.any.keys('a', 'b');
     *
     * The aliases `.includes`, `.contain`, and `.contains` can be used
     * interchangeably with `.include`.
     *
     * @name include
     * @alias contain
     * @alias includes
     * @alias contains
     * @param {Mixed} val
     * @param {String} msg _optional_
     * @namespace BDD
     * @api public
     */
  
    function SameValueZero(a, b) {
      return (_.isNaN(a) && _.isNaN(b)) || a === b;
    }
  
    function includeChainingBehavior () {
      flag(this, 'contains', true);
    }
  
    function include (val, msg) {
      if (msg) flag(this, 'message', msg);
  
      var obj = flag(this, 'object')
        , objType = _.type(obj).toLowerCase()
        , flagMsg = flag(this, 'message')
        , negate = flag(this, 'negate')
        , ssfi = flag(this, 'ssfi')
        , isDeep = flag(this, 'deep')
        , descriptor = isDeep ? 'deep ' : '';
  
      flagMsg = flagMsg ? flagMsg + ': ' : '';
  
      var included = false;
  
      switch (objType) {
        case 'string':
          included = obj.indexOf(val) !== -1;
          break;
  
        case 'weakset':
          if (isDeep) {
            throw new AssertionError(
              flagMsg + 'unable to use .deep.include with WeakSet',
              undefined,
              ssfi
            );
          }
  
          included = obj.has(val);
          break;
  
        case 'map':
          var isEql = isDeep ? _.eql : SameValueZero;
          obj.forEach(function (item) {
            included = included || isEql(item, val);
          });
          break;
  
        case 'set':
          if (isDeep) {
            obj.forEach(function (item) {
              included = included || _.eql(item, val);
            });
          } else {
            included = obj.has(val);
          }
          break;
  
        case 'array':
          if (isDeep) {
            included = obj.some(function (item) {
              return _.eql(item, val);
            })
          } else {
            included = obj.indexOf(val) !== -1;
          }
          break;
  
        default:
          // This block is for asserting a subset of properties in an object.
          // `_.expectTypes` isn't used here because `.include` should work with
          // objects with a custom `@@toStringTag`.
          if (val !== Object(val)) {
            throw new AssertionError(
              flagMsg + 'the given combination of arguments ('
              + objType + ' and '
              + _.type(val).toLowerCase() + ')'
              + ' is invalid for this assertion. '
              + 'You can use an array, a map, an object, a set, a string, '
              + 'or a weakset instead of a '
              + _.type(val).toLowerCase(),
              undefined,
              ssfi
            );
          }
  
          var props = Object.keys(val)
            , firstErr = null
            , numErrs = 0;
  
          props.forEach(function (prop) {
            var propAssertion = new Assertion(obj);
            _.transferFlags(this, propAssertion, true);
            flag(propAssertion, 'lockSsfi', true);
  
            if (!negate || props.length === 1) {
              propAssertion.property(prop, val[prop]);
              return;
            }
  
            try {
              propAssertion.property(prop, val[prop]);
            } catch (err) {
              if (!_.checkError.compatibleConstructor(err, AssertionError)) {
                throw err;
              }
              if (firstErr === null) firstErr = err;
              numErrs++;
            }
          }, this);
  
          // When validating .not.include with multiple properties, we only want
          // to throw an assertion error if all of the properties are included,
          // in which case we throw the first property assertion error that we
          // encountered.
          if (negate && props.length > 1 && numErrs === props.length) {
            throw firstErr;
          }
          return;
      }
  
      // Assert inclusion in collection or substring in a string.
      this.assert(
        included
        , 'expected #{this} to ' + descriptor + 'include ' + _.inspect(val)
        , 'expected #{this} to not ' + descriptor + 'include ' + _.inspect(val));
    }
  
    Assertion.addChainableMethod('include', include, includeChainingBehavior);
    Assertion.addChainableMethod('contain', include, includeChainingBehavior);
    Assertion.addChainableMethod('contains', include, includeChainingBehavior);
    Assertion.addChainableMethod('includes', include, includeChainingBehavior);
  
    /**
     * ### .ok
     *
     * Asserts that the target is a truthy value (considered `true` in boolean context).
     * However, it's often best to assert that the target is strictly (`===`) or
     * deeply equal to its expected value.
     *
     *     expect(1).to.equal(1); // Recommended
     *     expect(1).to.be.ok; // Not recommended
     *
     *     expect(true).to.be.true; // Recommended
     *     expect(true).to.be.ok; // Not recommended
     *
     * Add `.not` earlier in the chain to negate `.ok`.
     *
     *     expect(0).to.equal(0); // Recommended
     *     expect(0).to.not.be.ok; // Not recommended
     *
     *     expect(false).to.be.false; // Recommended
     *     expect(false).to.not.be.ok; // Not recommended
     *
     *     expect(null).to.be.null; // Recommended
     *     expect(null).to.not.be.ok; // Not recommended
     *
     *     expect(undefined).to.be.undefined; // Recommended
     *     expect(undefined).to.not.be.ok; // Not recommended
     *
     * A custom error message can be given as the second argument to `expect`.
     *
     *     expect(false, 'nooo why fail??').to.be.ok;
     *
     * @name ok
     * @namespace BDD
     * @api public
     */
  
    Assertion.addProperty('ok', function () {
      this.assert(
          flag(this, 'object')
        , 'expected #{this} to be truthy'
        , 'expected #{this} to be falsy');
    });
  
    /**
     * ### .true
     *
     * Asserts that the target is strictly (`===`) equal to `true`.
     *
     *     expect(true).to.be.true;
     *
     * Add `.not` earlier in the chain to negate `.true`. However, it's often best
     * to assert that the target is equal to its expected value, rather than not
     * equal to `true`.
     *
     *     expect(false).to.be.false; // Recommended
     *     expect(false).to.not.be.true; // Not recommended
     *
     *     expect(1).to.equal(1); // Recommended
     *     expect(1).to.not.be.true; // Not recommended
     *
     * A custom error message can be given as the second argument to `expect`.
     *
     *     expect(false, 'nooo why fail??').to.be.true;
     *
     * @name true
     * @namespace BDD
     * @api public
     */
  
    Assertion.addProperty('true', function () {
      this.assert(
          true === flag(this, 'object')
        , 'expected #{this} to be true'
        , 'expected #{this} to be false'
        , flag(this, 'negate') ? false : true
      );
    });
  
    /**
     * ### .false
     *
     * Asserts that the target is strictly (`===`) equal to `false`.
     *
     *     expect(false).to.be.false;
     *
     * Add `.not` earlier in the chain to negate `.false`. However, it's often
     * best to assert that the target is equal to its expected value, rather than
     * not equal to `false`.
     *
     *     expect(true).to.be.true; // Recommended
     *     expect(true).to.not.be.false; // Not recommended
     *
     *     expect(1).to.equal(1); // Recommended
     *     expect(1).to.not.be.false; // Not recommended
     *
     * A custom error message can be given as the second argument to `expect`.
     *
     *     expect(true, 'nooo why fail??').to.be.false;
     *
     * @name false
     * @namespace BDD
     * @api public
     */
  
    Assertion.addProperty('false', function () {
      this.assert(
          false === flag(this, 'object')
        , 'expected #{this} to be false'
        , 'expected #{this} to be true'
        , flag(this, 'negate') ? true : false
      );
    });
  
    /**
     * ### .null
     *
     * Asserts that the target is strictly (`===`) equal to `null`.
     *
     *     expect(null).to.be.null;
     *
     * Add `.not` earlier in the chain to negate `.null`. However, it's often best
     * to assert that the target is equal to its expected value, rather than not
     * equal to `null`.
     *
     *     expect(1).to.equal(1); // Recommended
     *     expect(1).to.not.be.null; // Not recommended
     *
     * A custom error message can be given as the second argument to `expect`.
     *
     *     expect(42, 'nooo why fail??').to.be.null;
     *
     * @name null
     * @namespace BDD
     * @api public
     */
  
    Assertion.addProperty('null', function () {
      this.assert(
          null === flag(this, 'object')
        , 'expected #{this} to be null'
        , 'expected #{this} not to be null'
      );
    });
  
    /**
     * ### .undefined
     *
     * Asserts that the target is strictly (`===`) equal to `undefined`.
     *
     *     expect(undefined).to.be.undefined;
     *
     * Add `.not` earlier in the chain to negate `.undefined`. However, it's often
     * best to assert that the target is equal to its expected value, rather than
     * not equal to `undefined`.
     *
     *     expect(1).to.equal(1); // Recommended
     *     expect(1).to.not.be.undefined; // Not recommended
     *
     * A custom error message can be given as the second argument to `expect`.
     *
     *     expect(42, 'nooo why fail??').to.be.undefined;
     *
     * @name undefined
     * @namespace BDD
     * @api public
     */
  
    Assertion.addProperty('undefined', function () {
      this.assert(
          undefined === flag(this, 'object')
        , 'expected #{this} to be undefined'
        , 'expected #{this} not to be undefined'
      );
    });
  
    /**
     * ### .NaN
     *
     * Asserts that the target is exactly `NaN`.
     *
     *     expect(NaN).to.be.NaN;
     *
     * Add `.not` earlier in the chain to negate `.NaN`. However, it's often best
     * to assert that the target is equal to its expected value, rather than not
     * equal to `NaN`.
     *
     *     expect('foo').to.equal('foo'); // Recommended
     *     expect('foo').to.not.be.NaN; // Not recommended
     *
     * A custom error message can be given as the second argument to `expect`.
     *
     *     expect(42, 'nooo why fail??').to.be.NaN;
     *
     * @name NaN
     * @namespace BDD
     * @api public
     */
  
    Assertion.addProperty('NaN', function () {
      this.assert(
          _.isNaN(flag(this, 'object'))
          , 'expected #{this} to be NaN'
          , 'expected #{this} not to be NaN'
      );
    });
  
    /**
     * ### .exist
     *
     * Asserts that the target is not strictly (`===`) equal to either `null` or
     * `undefined`. However, it's often best to assert that the target is equal to
     * its expected value.
     *
     *     expect(1).to.equal(1); // Recommended
     *     expect(1).to.exist; // Not recommended
     *
     *     expect(0).to.equal(0); // Recommended
     *     expect(0).to.exist; // Not recommended
     *
     * Add `.not` earlier in the chain to negate `.exist`.
     *
     *     expect(null).to.be.null; // Recommended
     *     expect(null).to.not.exist; // Not recommended
     *
     *     expect(undefined).to.be.undefined; // Recommended
     *     expect(undefined).to.not.exist; // Not recommended
     *
     * A custom error message can be given as the second argument to `expect`.
     *
     *     expect(null, 'nooo why fail??').to.exist;
     *
     * @name exist
     * @namespace BDD
     * @api public
     */
  
    Assertion.addProperty('exist', function () {
      var val = flag(this, 'object');
      this.assert(
          val !== null && val !== undefined
        , 'expected #{this} to exist'
        , 'expected #{this} to not exist'
      );
    });
  
    /**
     * ### .empty
     *
     * When the target is a string or array, `.empty` asserts that the target's
     * `length` property is strictly (`===`) equal to `0`.
     *
     *     expect([]).to.be.empty;
     *     expect('').to.be.empty;
     *
     * When the target is a map or set, `.empty` asserts that the target's `size`
     * property is strictly equal to `0`.
     *
     *     expect(new Set()).to.be.empty;
     *     expect(new Map()).to.be.empty;
     *
     * When the target is a non-function object, `.empty` asserts that the target
     * doesn't have any own enumerable properties. Properties with Symbol-based
     * keys are excluded from the count.
     *
     *     expect({}).to.be.empty;
     *
     * Because `.empty` does different things based on the target's type, it's
     * important to check the target's type before using `.empty`. See the `.a`
     * doc for info on testing a target's type.
     *
     *     expect([]).to.be.an('array').that.is.empty;
     *
     * Add `.not` earlier in the chain to negate `.empty`. However, it's often
     * best to assert that the target contains its expected number of values,
     * rather than asserting that it's not empty.
     *
     *     expect([1, 2, 3]).to.have.lengthOf(3); // Recommended
     *     expect([1, 2, 3]).to.not.be.empty; // Not recommended
     *
     *     expect(new Set([1, 2, 3])).to.have.property('size', 3); // Recommended
     *     expect(new Set([1, 2, 3])).to.not.be.empty; // Not recommended
     *
     *     expect(Object.keys({a: 1})).to.have.lengthOf(1); // Recommended
     *     expect({a: 1}).to.not.be.empty; // Not recommended
     *
     * A custom error message can be given as the second argument to `expect`.
     *
     *     expect([1, 2, 3], 'nooo why fail??').to.be.empty;
     *
     * @name empty
     * @namespace BDD
     * @api public
     */
  
    Assertion.addProperty('empty', function () {
      var val = flag(this, 'object')
        , ssfi = flag(this, 'ssfi')
        , flagMsg = flag(this, 'message')
        , itemsCount;
  
      flagMsg = flagMsg ? flagMsg + ': ' : '';
  
      switch (_.type(val).toLowerCase()) {
        case 'array':
        case 'string':
          itemsCount = val.length;
          break;
        case 'map':
        case 'set':
          itemsCount = val.size;
          break;
        case 'weakmap':
        case 'weakset':
          throw new AssertionError(
            flagMsg + '.empty was passed a weak collection',
            undefined,
            ssfi
          );
        case 'function':
          var msg = flagMsg + '.empty was passed a function ' + _.getName(val);
          throw new AssertionError(msg.trim(), undefined, ssfi);
        default:
          if (val !== Object(val)) {
            throw new AssertionError(
              flagMsg + '.empty was passed non-string primitive ' + _.inspect(val),
              undefined,
              ssfi
            );
          }
          itemsCount = Object.keys(val).length;
      }
  
      this.assert(
          0 === itemsCount
        , 'expected #{this} to be empty'
        , 'expected #{this} not to be empty'
      );
    });
  
    /**
     * ### .arguments
     *
     * Asserts that the target is an `arguments` object.
     *
     *     function test () {
     *       expect(arguments).to.be.arguments;
     *     }
     *
     *     test();
     *
     * Add `.not` earlier in the chain to negate `.arguments`. However, it's often
     * best to assert which type the target is expected to be, rather than
     * asserting that it’s not an `arguments` object.
     *
     *     expect('foo').to.be.a('string'); // Recommended
     *     expect('foo').to.not.be.arguments; // Not recommended
     *
     * A custom error message can be given as the second argument to `expect`.
     *
     *     expect({}, 'nooo why fail??').to.be.arguments;
     *
     * The alias `.Arguments` can be used interchangeably with `.arguments`.
     *
     * @name arguments
     * @alias Arguments
     * @namespace BDD
     * @api public
     */
  
    function checkArguments () {
      var obj = flag(this, 'object')
        , type = _.type(obj);
      this.assert(
          'Arguments' === type
        , 'expected #{this} to be arguments but got ' + type
        , 'expected #{this} to not be arguments'
      );
    }
  
    Assertion.addProperty('arguments', checkArguments);
    Assertion.addProperty('Arguments', checkArguments);
  
    /**
     * ### .equal(val[, msg])
     *
     * Asserts that the target is strictly (`===`) equal to the given `val`.
     *
     *     expect(1).to.equal(1);
     *     expect('foo').to.equal('foo');
     *
     * Add `.deep` earlier in the chain to use deep equality instead. See the
     * `deep-eql` project page for info on the deep equality algorithm:
     * https://github.com/chaijs/deep-eql.
     *
     *     // Target object deeply (but not strictly) equals `{a: 1}`
     *     expect({a: 1}).to.deep.equal({a: 1});
     *     expect({a: 1}).to.not.equal({a: 1});
     *
     *     // Target array deeply (but not strictly) equals `[1, 2]`
     *     expect([1, 2]).to.deep.equal([1, 2]);
     *     expect([1, 2]).to.not.equal([1, 2]);
     *
     * Add `.not` earlier in the chain to negate `.equal`. However, it's often
     * best to assert that the target is equal to its expected value, rather than
     * not equal to one of countless unexpected values.
     *
     *     expect(1).to.equal(1); // Recommended
     *     expect(1).to.not.equal(2); // Not recommended
     *
     * `.equal` accepts an optional `msg` argument which is a custom error message
     * to show when the assertion fails. The message can also be given as the
     * second argument to `expect`.
     *
     *     expect(1).to.equal(2, 'nooo why fail??');
     *     expect(1, 'nooo why fail??').to.equal(2);
     *
     * The aliases `.equals` and `eq` can be used interchangeably with `.equal`.
     *
     * @name equal
     * @alias equals
     * @alias eq
     * @param {Mixed} val
     * @param {String} msg _optional_
     * @namespace BDD
     * @api public
     */
  
    function assertEqual (val, msg) {
      if (msg) flag(this, 'message', msg);
      var obj = flag(this, 'object');
      if (flag(this, 'deep')) {
        var prevLockSsfi = flag(this, 'lockSsfi');
        flag(this, 'lockSsfi', true);
        this.eql(val);
        flag(this, 'lockSsfi', prevLockSsfi);
      } else {
        this.assert(
            val === obj
          , 'expected #{this} to equal #{exp}'
          , 'expected #{this} to not equal #{exp}'
          , val
          , this._obj
          , true
        );
      }
    }
  
    Assertion.addMethod('equal', assertEqual);
    Assertion.addMethod('equals', assertEqual);
    Assertion.addMethod('eq', assertEqual);
  
    /**
     * ### .eql(obj[, msg])
     *
     * Asserts that the target is deeply equal to the given `obj`. See the
     * `deep-eql` project page for info on the deep equality algorithm:
     * https://github.com/chaijs/deep-eql.
     *
     *     // Target object is deeply (but not strictly) equal to {a: 1}
     *     expect({a: 1}).to.eql({a: 1}).but.not.equal({a: 1});
     *
     *     // Target array is deeply (but not strictly) equal to [1, 2]
     *     expect([1, 2]).to.eql([1, 2]).but.not.equal([1, 2]);
     *
     * Add `.not` earlier in the chain to negate `.eql`. However, it's often best
     * to assert that the target is deeply equal to its expected value, rather
     * than not deeply equal to one of countless unexpected values.
     *
     *     expect({a: 1}).to.eql({a: 1}); // Recommended
     *     expect({a: 1}).to.not.eql({b: 2}); // Not recommended
     *
     * `.eql` accepts an optional `msg` argument which is a custom error message
     * to show when the assertion fails. The message can also be given as the
     * second argument to `expect`.
     *
     *     expect({a: 1}).to.eql({b: 2}, 'nooo why fail??');
     *     expect({a: 1}, 'nooo why fail??').to.eql({b: 2});
     *
     * The alias `.eqls` can be used interchangeably with `.eql`.
     *
     * The `.deep.equal` assertion is almost identical to `.eql` but with one
     * difference: `.deep.equal` causes deep equality comparisons to also be used
     * for any other assertions that follow in the chain.
     *
     * @name eql
     * @alias eqls
     * @param {Mixed} obj
     * @param {String} msg _optional_
     * @namespace BDD
     * @api public
     */
  
    function assertEql(obj, msg) {
      if (msg) flag(this, 'message', msg);
      this.assert(
          _.eql(obj, flag(this, 'object'))
        , 'expected #{this} to deeply equal #{exp}'
        , 'expected #{this} to not deeply equal #{exp}'
        , obj
        , this._obj
        , true
      );
    }
  
    Assertion.addMethod('eql', assertEql);
    Assertion.addMethod('eqls', assertEql);
  
    /**
     * ### .above(n[, msg])
     *
     * Asserts that the target is a number or a date greater than the given number or date `n` respectively.
     * However, it's often best to assert that the target is equal to its expected
     * value.
     *
     *     expect(2).to.equal(2); // Recommended
     *     expect(2).to.be.above(1); // Not recommended
     *
     * Add `.lengthOf` earlier in the chain to assert that the target's `length`
     * or `size` is greater than the given number `n`.
     *
     *     expect('foo').to.have.lengthOf(3); // Recommended
     *     expect('foo').to.have.lengthOf.above(2); // Not recommended
     *
     *     expect([1, 2, 3]).to.have.lengthOf(3); // Recommended
     *     expect([1, 2, 3]).to.have.lengthOf.above(2); // Not recommended
     *
     * Add `.not` earlier in the chain to negate `.above`.
     *
     *     expect(2).to.equal(2); // Recommended
     *     expect(1).to.not.be.above(2); // Not recommended
     *
     * `.above` accepts an optional `msg` argument which is a custom error message
     * to show when the assertion fails. The message can also be given as the
     * second argument to `expect`.
     *
     *     expect(1).to.be.above(2, 'nooo why fail??');
     *     expect(1, 'nooo why fail??').to.be.above(2);
     *
     * The aliases `.gt` and `.greaterThan` can be used interchangeably with
     * `.above`.
     *
     * @name above
     * @alias gt
     * @alias greaterThan
     * @param {Number} n
     * @param {String} msg _optional_
     * @namespace BDD
     * @api public
     */
  
    function assertAbove (n, msg) {
      if (msg) flag(this, 'message', msg);
      var obj = flag(this, 'object')
        , doLength = flag(this, 'doLength')
        , flagMsg = flag(this, 'message')
        , msgPrefix = ((flagMsg) ? flagMsg + ': ' : '')
        , ssfi = flag(this, 'ssfi')
        , objType = _.type(obj).toLowerCase()
        , nType = _.type(n).toLowerCase()
        , errorMessage
        , shouldThrow = true;
  
      if (doLength && objType !== 'map' && objType !== 'set') {
        new Assertion(obj, flagMsg, ssfi, true).to.have.property('length');
      }
  
      if (!doLength && (objType === 'date' && nType !== 'date')) {
        errorMessage = msgPrefix + 'the argument to above must be a date';
      } else if (nType !== 'number' && (doLength || objType === 'number')) {
        errorMessage = msgPrefix + 'the argument to above must be a number';
      } else if (!doLength && (objType !== 'date' && objType !== 'number')) {
        var printObj = (objType === 'string') ? "'" + obj + "'" : obj;
        errorMessage = msgPrefix + 'expected ' + printObj + ' to be a number or a date';
      } else {
        shouldThrow = false;
      }
  
      if (shouldThrow) {
        throw new AssertionError(errorMessage, undefined, ssfi);
      }
  
      if (doLength) {
        var descriptor = 'length'
          , itemsCount;
        if (objType === 'map' || objType === 'set') {
          descriptor = 'size';
          itemsCount = obj.size;
        } else {
          itemsCount = obj.length;
        }
        this.assert(
            itemsCount > n
          , 'expected #{this} to have a ' + descriptor + ' above #{exp} but got #{act}'
          , 'expected #{this} to not have a ' + descriptor + ' above #{exp}'
          , n
          , itemsCount
        );
      } else {
        this.assert(
            obj > n
          , 'expected #{this} to be above #{exp}'
          , 'expected #{this} to be at most #{exp}'
          , n
        );
      }
    }
  
    Assertion.addMethod('above', assertAbove);
    Assertion.addMethod('gt', assertAbove);
    Assertion.addMethod('greaterThan', assertAbove);
  
    /**
     * ### .least(n[, msg])
     *
     * Asserts that the target is a number or a date greater than or equal to the given
     * number or date `n` respectively. However, it's often best to assert that the target is equal to
     * its expected value.
     *
     *     expect(2).to.equal(2); // Recommended
     *     expect(2).to.be.at.least(1); // Not recommended
     *     expect(2).to.be.at.least(2); // Not recommended
     *
     * Add `.lengthOf` earlier in the chain to assert that the target's `length`
     * or `size` is greater than or equal to the given number `n`.
     *
     *     expect('foo').to.have.lengthOf(3); // Recommended
     *     expect('foo').to.have.lengthOf.at.least(2); // Not recommended
     *
     *     expect([1, 2, 3]).to.have.lengthOf(3); // Recommended
     *     expect([1, 2, 3]).to.have.lengthOf.at.least(2); // Not recommended
     *
     * Add `.not` earlier in the chain to negate `.least`.
     *
     *     expect(1).to.equal(1); // Recommended
     *     expect(1).to.not.be.at.least(2); // Not recommended
     *
     * `.least` accepts an optional `msg` argument which is a custom error message
     * to show when the assertion fails. The message can also be given as the
     * second argument to `expect`.
     *
     *     expect(1).to.be.at.least(2, 'nooo why fail??');
     *     expect(1, 'nooo why fail??').to.be.at.least(2);
     *
     * The alias `.gte` can be used interchangeably with `.least`.
     *
     * @name least
     * @alias gte
     * @param {Number} n
     * @param {String} msg _optional_
     * @namespace BDD
     * @api public
     */
  
    function assertLeast (n, msg) {
      if (msg) flag(this, 'message', msg);
      var obj = flag(this, 'object')
        , doLength = flag(this, 'doLength')
        , flagMsg = flag(this, 'message')
        , msgPrefix = ((flagMsg) ? flagMsg + ': ' : '')
        , ssfi = flag(this, 'ssfi')
        , objType = _.type(obj).toLowerCase()
        , nType = _.type(n).toLowerCase()
        , errorMessage
        , shouldThrow = true;
  
      if (doLength && objType !== 'map' && objType !== 'set') {
        new Assertion(obj, flagMsg, ssfi, true).to.have.property('length');
      }
  
      if (!doLength && (objType === 'date' && nType !== 'date')) {
        errorMessage = msgPrefix + 'the argument to least must be a date';
      } else if (nType !== 'number' && (doLength || objType === 'number')) {
        errorMessage = msgPrefix + 'the argument to least must be a number';
      } else if (!doLength && (objType !== 'date' && objType !== 'number')) {
        var printObj = (objType === 'string') ? "'" + obj + "'" : obj;
        errorMessage = msgPrefix + 'expected ' + printObj + ' to be a number or a date';
      } else {
        shouldThrow = false;
      }
  
      if (shouldThrow) {
        throw new AssertionError(errorMessage, undefined, ssfi);
      }
  
      if (doLength) {
        var descriptor = 'length'
          , itemsCount;
        if (objType === 'map' || objType === 'set') {
          descriptor = 'size';
          itemsCount = obj.size;
        } else {
          itemsCount = obj.length;
        }
        this.assert(
            itemsCount >= n
          , 'expected #{this} to have a ' + descriptor + ' at least #{exp} but got #{act}'
          , 'expected #{this} to have a ' + descriptor + ' below #{exp}'
          , n
          , itemsCount
        );
      } else {
        this.assert(
            obj >= n
          , 'expected #{this} to be at least #{exp}'
          , 'expected #{this} to be below #{exp}'
          , n
        );
      }
    }
  
    Assertion.addMethod('least', assertLeast);
    Assertion.addMethod('gte', assertLeast);
  
    /**
     * ### .below(n[, msg])
     *
     * Asserts that the target is a number or a date less than the given number or date `n` respectively.
     * However, it's often best to assert that the target is equal to its expected
     * value.
     *
     *     expect(1).to.equal(1); // Recommended
     *     expect(1).to.be.below(2); // Not recommended
     *
     * Add `.lengthOf` earlier in the chain to assert that the target's `length`
     * or `size` is less than the given number `n`.
     *
     *     expect('foo').to.have.lengthOf(3); // Recommended
     *     expect('foo').to.have.lengthOf.below(4); // Not recommended
     *
     *     expect([1, 2, 3]).to.have.length(3); // Recommended
     *     expect([1, 2, 3]).to.have.lengthOf.below(4); // Not recommended
     *
     * Add `.not` earlier in the chain to negate `.below`.
     *
     *     expect(2).to.equal(2); // Recommended
     *     expect(2).to.not.be.below(1); // Not recommended
     *
     * `.below` accepts an optional `msg` argument which is a custom error message
     * to show when the assertion fails. The message can also be given as the
     * second argument to `expect`.
     *
     *     expect(2).to.be.below(1, 'nooo why fail??');
     *     expect(2, 'nooo why fail??').to.be.below(1);
     *
     * The aliases `.lt` and `.lessThan` can be used interchangeably with
     * `.below`.
     *
     * @name below
     * @alias lt
     * @alias lessThan
     * @param {Number} n
     * @param {String} msg _optional_
     * @namespace BDD
     * @api public
     */
  
    function assertBelow (n, msg) {
      if (msg) flag(this, 'message', msg);
      var obj = flag(this, 'object')
        , doLength = flag(this, 'doLength')
        , flagMsg = flag(this, 'message')
        , msgPrefix = ((flagMsg) ? flagMsg + ': ' : '')
        , ssfi = flag(this, 'ssfi')
        , objType = _.type(obj).toLowerCase()
        , nType = _.type(n).toLowerCase()
        , errorMessage
        , shouldThrow = true;
  
      if (doLength && objType !== 'map' && objType !== 'set') {
        new Assertion(obj, flagMsg, ssfi, true).to.have.property('length');
      }
  
      if (!doLength && (objType === 'date' && nType !== 'date')) {
        errorMessage = msgPrefix + 'the argument to below must be a date';
      } else if (nType !== 'number' && (doLength || objType === 'number')) {
        errorMessage = msgPrefix + 'the argument to below must be a number';
      } else if (!doLength && (objType !== 'date' && objType !== 'number')) {
        var printObj = (objType === 'string') ? "'" + obj + "'" : obj;
        errorMessage = msgPrefix + 'expected ' + printObj + ' to be a number or a date';
      } else {
        shouldThrow = false;
      }
  
      if (shouldThrow) {
        throw new AssertionError(errorMessage, undefined, ssfi);
      }
  
      if (doLength) {
        var descriptor = 'length'
          , itemsCount;
        if (objType === 'map' || objType === 'set') {
          descriptor = 'size';
          itemsCount = obj.size;
        } else {
          itemsCount = obj.length;
        }
        this.assert(
            itemsCount < n
          , 'expected #{this} to have a ' + descriptor + ' below #{exp} but got #{act}'
          , 'expected #{this} to not have a ' + descriptor + ' below #{exp}'
          , n
          , itemsCount
        );
      } else {
        this.assert(
            obj < n
          , 'expected #{this} to be below #{exp}'
          , 'expected #{this} to be at least #{exp}'
          , n
        );
      }
    }
  
    Assertion.addMethod('below', assertBelow);
    Assertion.addMethod('lt', assertBelow);
    Assertion.addMethod('lessThan', assertBelow);
  
    /**
     * ### .most(n[, msg])
     *
     * Asserts that the target is a number or a date less than or equal to the given number
     * or date `n` respectively. However, it's often best to assert that the target is equal to its
     * expected value.
     *
     *     expect(1).to.equal(1); // Recommended
     *     expect(1).to.be.at.most(2); // Not recommended
     *     expect(1).to.be.at.most(1); // Not recommended
     *
     * Add `.lengthOf` earlier in the chain to assert that the target's `length`
     * or `size` is less than or equal to the given number `n`.
     *
     *     expect('foo').to.have.lengthOf(3); // Recommended
     *     expect('foo').to.have.lengthOf.at.most(4); // Not recommended
     *
     *     expect([1, 2, 3]).to.have.lengthOf(3); // Recommended
     *     expect([1, 2, 3]).to.have.lengthOf.at.most(4); // Not recommended
     *
     * Add `.not` earlier in the chain to negate `.most`.
     *
     *     expect(2).to.equal(2); // Recommended
     *     expect(2).to.not.be.at.most(1); // Not recommended
     *
     * `.most` accepts an optional `msg` argument which is a custom error message
     * to show when the assertion fails. The message can also be given as the
     * second argument to `expect`.
     *
     *     expect(2).to.be.at.most(1, 'nooo why fail??');
     *     expect(2, 'nooo why fail??').to.be.at.most(1);
     *
     * The alias `.lte` can be used interchangeably with `.most`.
     *
     * @name most
     * @alias lte
     * @param {Number} n
     * @param {String} msg _optional_
     * @namespace BDD
     * @api public
     */
  
    function assertMost (n, msg) {
      if (msg) flag(this, 'message', msg);
      var obj = flag(this, 'object')
        , doLength = flag(this, 'doLength')
        , flagMsg = flag(this, 'message')
        , msgPrefix = ((flagMsg) ? flagMsg + ': ' : '')
        , ssfi = flag(this, 'ssfi')
        , objType = _.type(obj).toLowerCase()
        , nType = _.type(n).toLowerCase()
        , errorMessage
        , shouldThrow = true;
  
      if (doLength && objType !== 'map' && objType !== 'set') {
        new Assertion(obj, flagMsg, ssfi, true).to.have.property('length');
      }
  
      if (!doLength && (objType === 'date' && nType !== 'date')) {
        errorMessage = msgPrefix + 'the argument to most must be a date';
      } else if (nType !== 'number' && (doLength || objType === 'number')) {
        errorMessage = msgPrefix + 'the argument to most must be a number';
      } else if (!doLength && (objType !== 'date' && objType !== 'number')) {
        var printObj = (objType === 'string') ? "'" + obj + "'" : obj;
        errorMessage = msgPrefix + 'expected ' + printObj + ' to be a number or a date';
      } else {
        shouldThrow = false;
      }
  
      if (shouldThrow) {
        throw new AssertionError(errorMessage, undefined, ssfi);
      }
  
      if (doLength) {
        var descriptor = 'length'
          , itemsCount;
        if (objType === 'map' || objType === 'set') {
          descriptor = 'size';
          itemsCount = obj.size;
        } else {
          itemsCount = obj.length;
        }
        this.assert(
            itemsCount <= n
          , 'expected #{this} to have a ' + descriptor + ' at most #{exp} but got #{act}'
          , 'expected #{this} to have a ' + descriptor + ' above #{exp}'
          , n
          , itemsCount
        );
      } else {
        this.assert(
            obj <= n
          , 'expected #{this} to be at most #{exp}'
          , 'expected #{this} to be above #{exp}'
          , n
        );
      }
    }
  
    Assertion.addMethod('most', assertMost);
    Assertion.addMethod('lte', assertMost);
  
    /**
     * ### .within(start, finish[, msg])
     *
     * Asserts that the target is a number or a date greater than or equal to the given
     * number or date `start`, and less than or equal to the given number or date `finish` respectively.
     * However, it's often best to assert that the target is equal to its expected
     * value.
     *
     *     expect(2).to.equal(2); // Recommended
     *     expect(2).to.be.within(1, 3); // Not recommended
     *     expect(2).to.be.within(2, 3); // Not recommended
     *     expect(2).to.be.within(1, 2); // Not recommended
     *
     * Add `.lengthOf` earlier in the chain to assert that the target's `length`
     * or `size` is greater than or equal to the given number `start`, and less
     * than or equal to the given number `finish`.
     *
     *     expect('foo').to.have.lengthOf(3); // Recommended
     *     expect('foo').to.have.lengthOf.within(2, 4); // Not recommended
     *
     *     expect([1, 2, 3]).to.have.lengthOf(3); // Recommended
     *     expect([1, 2, 3]).to.have.lengthOf.within(2, 4); // Not recommended
     *
     * Add `.not` earlier in the chain to negate `.within`.
     *
     *     expect(1).to.equal(1); // Recommended
     *     expect(1).to.not.be.within(2, 4); // Not recommended
     *
     * `.within` accepts an optional `msg` argument which is a custom error
     * message to show when the assertion fails. The message can also be given as
     * the second argument to `expect`.
     *
     *     expect(4).to.be.within(1, 3, 'nooo why fail??');
     *     expect(4, 'nooo why fail??').to.be.within(1, 3);
     *
     * @name within
     * @param {Number} start lower bound inclusive
     * @param {Number} finish upper bound inclusive
     * @param {String} msg _optional_
     * @namespace BDD
     * @api public
     */
  
    Assertion.addMethod('within', function (start, finish, msg) {
      if (msg) flag(this, 'message', msg);
      var obj = flag(this, 'object')
        , doLength = flag(this, 'doLength')
        , flagMsg = flag(this, 'message')
        , msgPrefix = ((flagMsg) ? flagMsg + ': ' : '')
        , ssfi = flag(this, 'ssfi')
        , objType = _.type(obj).toLowerCase()
        , startType = _.type(start).toLowerCase()
        , finishType = _.type(finish).toLowerCase()
        , errorMessage
        , shouldThrow = true
        , range = (startType === 'date' && finishType === 'date')
            ? start.toUTCString() + '..' + finish.toUTCString()
            : start + '..' + finish;
  
      if (doLength && objType !== 'map' && objType !== 'set') {
        new Assertion(obj, flagMsg, ssfi, true).to.have.property('length');
      }
  
      if (!doLength && (objType === 'date' && (startType !== 'date' || finishType !== 'date'))) {
        errorMessage = msgPrefix + 'the arguments to within must be dates';
      } else if ((startType !== 'number' || finishType !== 'number') && (doLength || objType === 'number')) {
        errorMessage = msgPrefix + 'the arguments to within must be numbers';
      } else if (!doLength && (objType !== 'date' && objType !== 'number')) {
        var printObj = (objType === 'string') ? "'" + obj + "'" : obj;
        errorMessage = msgPrefix + 'expected ' + printObj + ' to be a number or a date';
      } else {
        shouldThrow = false;
      }
  
      if (shouldThrow) {
        throw new AssertionError(errorMessage, undefined, ssfi);
      }
  
      if (doLength) {
        var descriptor = 'length'
          , itemsCount;
        if (objType === 'map' || objType === 'set') {
          descriptor = 'size';
          itemsCount = obj.size;
        } else {
          itemsCount = obj.length;
        }
        this.assert(
            itemsCount >= start && itemsCount <= finish
          , 'expected #{this} to have a ' + descriptor + ' within ' + range
          , 'expected #{this} to not have a ' + descriptor + ' within ' + range
        );
      } else {
        this.assert(
            obj >= start && obj <= finish
          , 'expected #{this} to be within ' + range
          , 'expected #{this} to not be within ' + range
        );
      }
    });
  
    /**
     * ### .instanceof(constructor[, msg])
     *
     * Asserts that the target is an instance of the given `constructor`.
     *
     *     function Cat () { }
     *
     *     expect(new Cat()).to.be.an.instanceof(Cat);
     *     expect([1, 2]).to.be.an.instanceof(Array);
     *
     * Add `.not` earlier in the chain to negate `.instanceof`.
     *
     *     expect({a: 1}).to.not.be.an.instanceof(Array);
     *
     * `.instanceof` accepts an optional `msg` argument which is a custom error
     * message to show when the assertion fails. The message can also be given as
     * the second argument to `expect`.
     *
     *     expect(1).to.be.an.instanceof(Array, 'nooo why fail??');
     *     expect(1, 'nooo why fail??').to.be.an.instanceof(Array);
     *
     * Due to limitations in ES5, `.instanceof` may not always work as expected
     * when using a transpiler such as Babel or TypeScript. In particular, it may
     * produce unexpected results when subclassing built-in object such as
     * `Array`, `Error`, and `Map`. See your transpiler's docs for details:
     *
     * - ([Babel](https://babeljs.io/docs/usage/caveats/#classes))
     * - ([TypeScript](https://github.com/Microsoft/TypeScript/wiki/Breaking-Changes#extending-built-ins-like-error-array-and-map-may-no-longer-work))
     *
     * The alias `.instanceOf` can be used interchangeably with `.instanceof`.
     *
     * @name instanceof
     * @param {Constructor} constructor
     * @param {String} msg _optional_
     * @alias instanceOf
     * @namespace BDD
     * @api public
     */
  
    function assertInstanceOf (constructor, msg) {
      if (msg) flag(this, 'message', msg);
  
      var target = flag(this, 'object')
      var ssfi = flag(this, 'ssfi');
      var flagMsg = flag(this, 'message');
  
      try {
        var isInstanceOf = target instanceof constructor;
      } catch (err) {
        if (err instanceof TypeError) {
          flagMsg = flagMsg ? flagMsg + ': ' : '';
          throw new AssertionError(
            flagMsg + 'The instanceof assertion needs a constructor but '
              + _.type(constructor) + ' was given.',
            undefined,
            ssfi
          );
        }
        throw err;
      }
  
      var name = _.getName(constructor);
      if (name === null) {
        name = 'an unnamed constructor';
      }
  
      this.assert(
          isInstanceOf
        , 'expected #{this} to be an instance of ' + name
        , 'expected #{this} to not be an instance of ' + name
      );
    };
  
    Assertion.addMethod('instanceof', assertInstanceOf);
    Assertion.addMethod('instanceOf', assertInstanceOf);
  
    /**
     * ### .property(name[, val[, msg]])
     *
     * Asserts that the target has a property with the given key `name`.
     *
     *     expect({a: 1}).to.have.property('a');
     *
     * When `val` is provided, `.property` also asserts that the property's value
     * is equal to the given `val`.
     *
     *     expect({a: 1}).to.have.property('a', 1);
     *
     * By default, strict (`===`) equality is used. Add `.deep` earlier in the
     * chain to use deep equality instead. See the `deep-eql` project page for
     * info on the deep equality algorithm: https://github.com/chaijs/deep-eql.
     *
     *     // Target object deeply (but not strictly) has property `x: {a: 1}`
     *     expect({x: {a: 1}}).to.have.deep.property('x', {a: 1});
     *     expect({x: {a: 1}}).to.not.have.property('x', {a: 1});
     *
     * The target's enumerable and non-enumerable properties are always included
     * in the search. By default, both own and inherited properties are included.
     * Add `.own` earlier in the chain to exclude inherited properties from the
     * search.
     *
     *     Object.prototype.b = 2;
     *
     *     expect({a: 1}).to.have.own.property('a');
     *     expect({a: 1}).to.have.own.property('a', 1);
     *     expect({a: 1}).to.have.property('b');
     *     expect({a: 1}).to.not.have.own.property('b');
     *
     * `.deep` and `.own` can be combined.
     *
     *     expect({x: {a: 1}}).to.have.deep.own.property('x', {a: 1});
     *
     * Add `.nested` earlier in the chain to enable dot- and bracket-notation when
     * referencing nested properties.
     *
     *     expect({a: {b: ['x', 'y']}}).to.have.nested.property('a.b[1]');
     *     expect({a: {b: ['x', 'y']}}).to.have.nested.property('a.b[1]', 'y');
     *
     * If `.` or `[]` are part of an actual property name, they can be escaped by
     * adding two backslashes before them.
     *
     *     expect({'.a': {'[b]': 'x'}}).to.have.nested.property('\\.a.\\[b\\]');
     *
     * `.deep` and `.nested` can be combined.
     *
     *     expect({a: {b: [{c: 3}]}})
     *       .to.have.deep.nested.property('a.b[0]', {c: 3});
     *
     * `.own` and `.nested` cannot be combined.
     *
     * Add `.not` earlier in the chain to negate `.property`.
     *
     *     expect({a: 1}).to.not.have.property('b');
     *
     * However, it's dangerous to negate `.property` when providing `val`. The
     * problem is that it creates uncertain expectations by asserting that the
     * target either doesn't have a property with the given key `name`, or that it
     * does have a property with the given key `name` but its value isn't equal to
     * the given `val`. It's often best to identify the exact output that's
     * expected, and then write an assertion that only accepts that exact output.
     *
     * When the target isn't expected to have a property with the given key
     * `name`, it's often best to assert exactly that.
     *
     *     expect({b: 2}).to.not.have.property('a'); // Recommended
     *     expect({b: 2}).to.not.have.property('a', 1); // Not recommended
     *
     * When the target is expected to have a property with the given key `name`,
     * it's often best to assert that the property has its expected value, rather
     * than asserting that it doesn't have one of many unexpected values.
     *
     *     expect({a: 3}).to.have.property('a', 3); // Recommended
     *     expect({a: 3}).to.not.have.property('a', 1); // Not recommended
     *
     * `.property` changes the target of any assertions that follow in the chain
     * to be the value of the property from the original target object.
     *
     *     expect({a: 1}).to.have.property('a').that.is.a('number');
     *
     * `.property` accepts an optional `msg` argument which is a custom error
     * message to show when the assertion fails. The message can also be given as
     * the second argument to `expect`. When not providing `val`, only use the
     * second form.
     *
     *     // Recommended
     *     expect({a: 1}).to.have.property('a', 2, 'nooo why fail??');
     *     expect({a: 1}, 'nooo why fail??').to.have.property('a', 2);
     *     expect({a: 1}, 'nooo why fail??').to.have.property('b');
     *
     *     // Not recommended
     *     expect({a: 1}).to.have.property('b', undefined, 'nooo why fail??');
     *
     * The above assertion isn't the same thing as not providing `val`. Instead,
     * it's asserting that the target object has a `b` property that's equal to
     * `undefined`.
     *
     * The assertions `.ownProperty` and `.haveOwnProperty` can be used
     * interchangeably with `.own.property`.
     *
     * @name property
     * @param {String} name
     * @param {Mixed} val (optional)
     * @param {String} msg _optional_
     * @returns value of property for chaining
     * @namespace BDD
     * @api public
     */
  
    function assertProperty (name, val, msg) {
      if (msg) flag(this, 'message', msg);
  
      var isNested = flag(this, 'nested')
        , isOwn = flag(this, 'own')
        , flagMsg = flag(this, 'message')
        , obj = flag(this, 'object')
        , ssfi = flag(this, 'ssfi')
        , nameType = typeof name;
  
      flagMsg = flagMsg ? flagMsg + ': ' : '';
  
      if (isNested) {
        if (nameType !== 'string') {
          throw new AssertionError(
            flagMsg + 'the argument to property must be a string when using nested syntax',
            undefined,
            ssfi
          );
        }
      } else {
        if (nameType !== 'string' && nameType !== 'number' && nameType !== 'symbol') {
          throw new AssertionError(
            flagMsg + 'the argument to property must be a string, number, or symbol',
            undefined,
            ssfi
          );
        }
      }
  
      if (isNested && isOwn) {
        throw new AssertionError(
          flagMsg + 'The "nested" and "own" flags cannot be combined.',
          undefined,
          ssfi
        );
      }
  
      if (obj === null || obj === undefined) {
        throw new AssertionError(
          flagMsg + 'Target cannot be null or undefined.',
          undefined,
          ssfi
        );
      }
  
      var isDeep = flag(this, 'deep')
        , negate = flag(this, 'negate')
        , pathInfo = isNested ? _.getPathInfo(obj, name) : null
        , value = isNested ? pathInfo.value : obj[name];
  
      var descriptor = '';
      if (isDeep) descriptor += 'deep ';
      if (isOwn) descriptor += 'own ';
      if (isNested) descriptor += 'nested ';
      descriptor += 'property ';
  
      var hasProperty;
      if (isOwn) hasProperty = Object.prototype.hasOwnProperty.call(obj, name);
      else if (isNested) hasProperty = pathInfo.exists;
      else hasProperty = _.hasProperty(obj, name);
  
      // When performing a negated assertion for both name and val, merely having
      // a property with the given name isn't enough to cause the assertion to
      // fail. It must both have a property with the given name, and the value of
      // that property must equal the given val. Therefore, skip this assertion in
      // favor of the next.
      if (!negate || arguments.length === 1) {
        this.assert(
            hasProperty
          , 'expected #{this} to have ' + descriptor + _.inspect(name)
          , 'expected #{this} to not have ' + descriptor + _.inspect(name));
      }
  
      if (arguments.length > 1) {
        this.assert(
            hasProperty && (isDeep ? _.eql(val, value) : val === value)
          , 'expected #{this} to have ' + descriptor + _.inspect(name) + ' of #{exp}, but got #{act}'
          , 'expected #{this} to not have ' + descriptor + _.inspect(name) + ' of #{act}'
          , val
          , value
        );
      }
  
      flag(this, 'object', value);
    }
  
    Assertion.addMethod('property', assertProperty);
  
    function assertOwnProperty (name, value, msg) {
      flag(this, 'own', true);
      assertProperty.apply(this, arguments);
    }
  
    Assertion.addMethod('ownProperty', assertOwnProperty);
    Assertion.addMethod('haveOwnProperty', assertOwnProperty);
  
    /**
     * ### .ownPropertyDescriptor(name[, descriptor[, msg]])
     *
     * Asserts that the target has its own property descriptor with the given key
     * `name`. Enumerable and non-enumerable properties are included in the
     * search.
     *
     *     expect({a: 1}).to.have.ownPropertyDescriptor('a');
     *
     * When `descriptor` is provided, `.ownPropertyDescriptor` also asserts that
     * the property's descriptor is deeply equal to the given `descriptor`. See
     * the `deep-eql` project page for info on the deep equality algorithm:
     * https://github.com/chaijs/deep-eql.
     *
     *     expect({a: 1}).to.have.ownPropertyDescriptor('a', {
     *       configurable: true,
     *       enumerable: true,
     *       writable: true,
     *       value: 1,
     *     });
     *
     * Add `.not` earlier in the chain to negate `.ownPropertyDescriptor`.
     *
     *     expect({a: 1}).to.not.have.ownPropertyDescriptor('b');
     *
     * However, it's dangerous to negate `.ownPropertyDescriptor` when providing
     * a `descriptor`. The problem is that it creates uncertain expectations by
     * asserting that the target either doesn't have a property descriptor with
     * the given key `name`, or that it does have a property descriptor with the
     * given key `name` but it’s not deeply equal to the given `descriptor`. It's
     * often best to identify the exact output that's expected, and then write an
     * assertion that only accepts that exact output.
     *
     * When the target isn't expected to have a property descriptor with the given
     * key `name`, it's often best to assert exactly that.
     *
     *     // Recommended
     *     expect({b: 2}).to.not.have.ownPropertyDescriptor('a');
     *
     *     // Not recommended
     *     expect({b: 2}).to.not.have.ownPropertyDescriptor('a', {
     *       configurable: true,
     *       enumerable: true,
     *       writable: true,
     *       value: 1,
     *     });
     *
     * When the target is expected to have a property descriptor with the given
     * key `name`, it's often best to assert that the property has its expected
     * descriptor, rather than asserting that it doesn't have one of many
     * unexpected descriptors.
     *
     *     // Recommended
     *     expect({a: 3}).to.have.ownPropertyDescriptor('a', {
     *       configurable: true,
     *       enumerable: true,
     *       writable: true,
     *       value: 3,
     *     });
     *
     *     // Not recommended
     *     expect({a: 3}).to.not.have.ownPropertyDescriptor('a', {
     *       configurable: true,
     *       enumerable: true,
     *       writable: true,
     *       value: 1,
     *     });
     *
     * `.ownPropertyDescriptor` changes the target of any assertions that follow
     * in the chain to be the value of the property descriptor from the original
     * target object.
     *
     *     expect({a: 1}).to.have.ownPropertyDescriptor('a')
     *       .that.has.property('enumerable', true);
     *
     * `.ownPropertyDescriptor` accepts an optional `msg` argument which is a
     * custom error message to show when the assertion fails. The message can also
     * be given as the second argument to `expect`. When not providing
     * `descriptor`, only use the second form.
     *
     *     // Recommended
     *     expect({a: 1}).to.have.ownPropertyDescriptor('a', {
     *       configurable: true,
     *       enumerable: true,
     *       writable: true,
     *       value: 2,
     *     }, 'nooo why fail??');
     *
     *     // Recommended
     *     expect({a: 1}, 'nooo why fail??').to.have.ownPropertyDescriptor('a', {
     *       configurable: true,
     *       enumerable: true,
     *       writable: true,
     *       value: 2,
     *     });
     *
     *     // Recommended
     *     expect({a: 1}, 'nooo why fail??').to.have.ownPropertyDescriptor('b');
     *
     *     // Not recommended
     *     expect({a: 1})
     *       .to.have.ownPropertyDescriptor('b', undefined, 'nooo why fail??');
     *
     * The above assertion isn't the same thing as not providing `descriptor`.
     * Instead, it's asserting that the target object has a `b` property
     * descriptor that's deeply equal to `undefined`.
     *
     * The alias `.haveOwnPropertyDescriptor` can be used interchangeably with
     * `.ownPropertyDescriptor`.
     *
     * @name ownPropertyDescriptor
     * @alias haveOwnPropertyDescriptor
     * @param {String} name
     * @param {Object} descriptor _optional_
     * @param {String} msg _optional_
     * @namespace BDD
     * @api public
     */
  
    function assertOwnPropertyDescriptor (name, descriptor, msg) {
      if (typeof descriptor === 'string') {
        msg = descriptor;
        descriptor = null;
      }
      if (msg) flag(this, 'message', msg);
      var obj = flag(this, 'object');
      var actualDescriptor = Object.getOwnPropertyDescriptor(Object(obj), name);
      if (actualDescriptor && descriptor) {
        this.assert(
            _.eql(descriptor, actualDescriptor)
          , 'expected the own property descriptor for ' + _.inspect(name) + ' on #{this} to match ' + _.inspect(descriptor) + ', got ' + _.inspect(actualDescriptor)
          , 'expected the own property descriptor for ' + _.inspect(name) + ' on #{this} to not match ' + _.inspect(descriptor)
          , descriptor
          , actualDescriptor
          , true
        );
      } else {
        this.assert(
            actualDescriptor
          , 'expected #{this} to have an own property descriptor for ' + _.inspect(name)
          , 'expected #{this} to not have an own property descriptor for ' + _.inspect(name)
        );
      }
      flag(this, 'object', actualDescriptor);
    }
  
    Assertion.addMethod('ownPropertyDescriptor', assertOwnPropertyDescriptor);
    Assertion.addMethod('haveOwnPropertyDescriptor', assertOwnPropertyDescriptor);
  
    /**
     * ### .lengthOf(n[, msg])
     *
     * Asserts that the target's `length` or `size` is equal to the given number
     * `n`.
     *
     *     expect([1, 2, 3]).to.have.lengthOf(3);
     *     expect('foo').to.have.lengthOf(3);
     *     expect(new Set([1, 2, 3])).to.have.lengthOf(3);
     *     expect(new Map([['a', 1], ['b', 2], ['c', 3]])).to.have.lengthOf(3);
     *
     * Add `.not` earlier in the chain to negate `.lengthOf`. However, it's often
     * best to assert that the target's `length` property is equal to its expected
     * value, rather than not equal to one of many unexpected values.
     *
     *     expect('foo').to.have.lengthOf(3); // Recommended
     *     expect('foo').to.not.have.lengthOf(4); // Not recommended
     *
     * `.lengthOf` accepts an optional `msg` argument which is a custom error
     * message to show when the assertion fails. The message can also be given as
     * the second argument to `expect`.
     *
     *     expect([1, 2, 3]).to.have.lengthOf(2, 'nooo why fail??');
     *     expect([1, 2, 3], 'nooo why fail??').to.have.lengthOf(2);
     *
     * `.lengthOf` can also be used as a language chain, causing all `.above`,
     * `.below`, `.least`, `.most`, and `.within` assertions that follow in the
     * chain to use the target's `length` property as the target. However, it's
     * often best to assert that the target's `length` property is equal to its
     * expected length, rather than asserting that its `length` property falls
     * within some range of values.
     *
     *     // Recommended
     *     expect([1, 2, 3]).to.have.lengthOf(3);
     *
     *     // Not recommended
     *     expect([1, 2, 3]).to.have.lengthOf.above(2);
     *     expect([1, 2, 3]).to.have.lengthOf.below(4);
     *     expect([1, 2, 3]).to.have.lengthOf.at.least(3);
     *     expect([1, 2, 3]).to.have.lengthOf.at.most(3);
     *     expect([1, 2, 3]).to.have.lengthOf.within(2,4);
     *
     * Due to a compatibility issue, the alias `.length` can't be chained directly
     * off of an uninvoked method such as `.a`. Therefore, `.length` can't be used
     * interchangeably with `.lengthOf` in every situation. It's recommended to
     * always use `.lengthOf` instead of `.length`.
     *
     *     expect([1, 2, 3]).to.have.a.length(3); // incompatible; throws error
     *     expect([1, 2, 3]).to.have.a.lengthOf(3);  // passes as expected
     *
     * @name lengthOf
     * @alias length
     * @param {Number} n
     * @param {String} msg _optional_
     * @namespace BDD
     * @api public
     */
  
    function assertLengthChain () {
      flag(this, 'doLength', true);
    }
  
    function assertLength (n, msg) {
      if (msg) flag(this, 'message', msg);
      var obj = flag(this, 'object')
        , objType = _.type(obj).toLowerCase()
        , flagMsg = flag(this, 'message')
        , ssfi = flag(this, 'ssfi')
        , descriptor = 'length'
        , itemsCount;
  
      switch (objType) {
        case 'map':
        case 'set':
          descriptor = 'size';
          itemsCount = obj.size;
          break;
        default:
          new Assertion(obj, flagMsg, ssfi, true).to.have.property('length');
          itemsCount = obj.length;
      }
  
      this.assert(
          itemsCount == n
        , 'expected #{this} to have a ' + descriptor + ' of #{exp} but got #{act}'
        , 'expected #{this} to not have a ' + descriptor + ' of #{act}'
        , n
        , itemsCount
      );
    }
  
    Assertion.addChainableMethod('length', assertLength, assertLengthChain);
    Assertion.addChainableMethod('lengthOf', assertLength, assertLengthChain);
  
    /**
     * ### .match(re[, msg])
     *
     * Asserts that the target matches the given regular expression `re`.
     *
     *     expect('foobar').to.match(/^foo/);
     *
     * Add `.not` earlier in the chain to negate `.match`.
     *
     *     expect('foobar').to.not.match(/taco/);
     *
     * `.match` accepts an optional `msg` argument which is a custom error message
     * to show when the assertion fails. The message can also be given as the
     * second argument to `expect`.
     *
     *     expect('foobar').to.match(/taco/, 'nooo why fail??');
     *     expect('foobar', 'nooo why fail??').to.match(/taco/);
     *
     * The alias `.matches` can be used interchangeably with `.match`.
     *
     * @name match
     * @alias matches
     * @param {RegExp} re
     * @param {String} msg _optional_
     * @namespace BDD
     * @api public
     */
    function assertMatch(re, msg) {
      if (msg) flag(this, 'message', msg);
      var obj = flag(this, 'object');
      this.assert(
          re.exec(obj)
        , 'expected #{this} to match ' + re
        , 'expected #{this} not to match ' + re
      );
    }
  
    Assertion.addMethod('match', assertMatch);
    Assertion.addMethod('matches', assertMatch);
  
    /**
     * ### .string(str[, msg])
     *
     * Asserts that the target string contains the given substring `str`.
     *
     *     expect('foobar').to.have.string('bar');
     *
     * Add `.not` earlier in the chain to negate `.string`.
     *
     *     expect('foobar').to.not.have.string('taco');
     *
     * `.string` accepts an optional `msg` argument which is a custom error
     * message to show when the assertion fails. The message can also be given as
     * the second argument to `expect`.
     *
     *     expect('foobar').to.have.string('taco', 'nooo why fail??');
     *     expect('foobar', 'nooo why fail??').to.have.string('taco');
     *
     * @name string
     * @param {String} str
     * @param {String} msg _optional_
     * @namespace BDD
     * @api public
     */
  
    Assertion.addMethod('string', function (str, msg) {
      if (msg) flag(this, 'message', msg);
      var obj = flag(this, 'object')
        , flagMsg = flag(this, 'message')
        , ssfi = flag(this, 'ssfi');
      new Assertion(obj, flagMsg, ssfi, true).is.a('string');
  
      this.assert(
          ~obj.indexOf(str)
        , 'expected #{this} to contain ' + _.inspect(str)
        , 'expected #{this} to not contain ' + _.inspect(str)
      );
    });
  
    /**
     * ### .keys(key1[, key2[, ...]])
     *
     * Asserts that the target object, array, map, or set has the given keys. Only
     * the target's own inherited properties are included in the search.
     *
     * When the target is an object or array, keys can be provided as one or more
     * string arguments, a single array argument, or a single object argument. In
     * the latter case, only the keys in the given object matter; the values are
     * ignored.
     *
     *     expect({a: 1, b: 2}).to.have.all.keys('a', 'b');
     *     expect(['x', 'y']).to.have.all.keys(0, 1);
     *
     *     expect({a: 1, b: 2}).to.have.all.keys(['a', 'b']);
     *     expect(['x', 'y']).to.have.all.keys([0, 1]);
     *
     *     expect({a: 1, b: 2}).to.have.all.keys({a: 4, b: 5}); // ignore 4 and 5
     *     expect(['x', 'y']).to.have.all.keys({0: 4, 1: 5}); // ignore 4 and 5
     *
     * When the target is a map or set, each key must be provided as a separate
     * argument.
     *
     *     expect(new Map([['a', 1], ['b', 2]])).to.have.all.keys('a', 'b');
     *     expect(new Set(['a', 'b'])).to.have.all.keys('a', 'b');
     *
     * Because `.keys` does different things based on the target's type, it's
     * important to check the target's type before using `.keys`. See the `.a` doc
     * for info on testing a target's type.
     *
     *     expect({a: 1, b: 2}).to.be.an('object').that.has.all.keys('a', 'b');
     *
     * By default, strict (`===`) equality is used to compare keys of maps and
     * sets. Add `.deep` earlier in the chain to use deep equality instead. See
     * the `deep-eql` project page for info on the deep equality algorithm:
     * https://github.com/chaijs/deep-eql.
     *
     *     // Target set deeply (but not strictly) has key `{a: 1}`
     *     expect(new Set([{a: 1}])).to.have.all.deep.keys([{a: 1}]);
     *     expect(new Set([{a: 1}])).to.not.have.all.keys([{a: 1}]);
     *
     * By default, the target must have all of the given keys and no more. Add
     * `.any` earlier in the chain to only require that the target have at least
     * one of the given keys. Also, add `.not` earlier in the chain to negate
     * `.keys`. It's often best to add `.any` when negating `.keys`, and to use
     * `.all` when asserting `.keys` without negation.
     *
     * When negating `.keys`, `.any` is preferred because `.not.any.keys` asserts
     * exactly what's expected of the output, whereas `.not.all.keys` creates
     * uncertain expectations.
     *
     *     // Recommended; asserts that target doesn't have any of the given keys
     *     expect({a: 1, b: 2}).to.not.have.any.keys('c', 'd');
     *
     *     // Not recommended; asserts that target doesn't have all of the given
     *     // keys but may or may not have some of them
     *     expect({a: 1, b: 2}).to.not.have.all.keys('c', 'd');
     *
     * When asserting `.keys` without negation, `.all` is preferred because
     * `.all.keys` asserts exactly what's expected of the output, whereas
     * `.any.keys` creates uncertain expectations.
     *
     *     // Recommended; asserts that target has all the given keys
     *     expect({a: 1, b: 2}).to.have.all.keys('a', 'b');
     *
     *     // Not recommended; asserts that target has at least one of the given
     *     // keys but may or may not have more of them
     *     expect({a: 1, b: 2}).to.have.any.keys('a', 'b');
     *
     * Note that `.all` is used by default when neither `.all` nor `.any` appear
     * earlier in the chain. However, it's often best to add `.all` anyway because
     * it improves readability.
     *
     *     // Both assertions are identical
     *     expect({a: 1, b: 2}).to.have.all.keys('a', 'b'); // Recommended
     *     expect({a: 1, b: 2}).to.have.keys('a', 'b'); // Not recommended
     *
     * Add `.include` earlier in the chain to require that the target's keys be a
     * superset of the expected keys, rather than identical sets.
     *
     *     // Target object's keys are a superset of ['a', 'b'] but not identical
     *     expect({a: 1, b: 2, c: 3}).to.include.all.keys('a', 'b');
     *     expect({a: 1, b: 2, c: 3}).to.not.have.all.keys('a', 'b');
     *
     * However, if `.any` and `.include` are combined, only the `.any` takes
     * effect. The `.include` is ignored in this case.
     *
     *     // Both assertions are identical
     *     expect({a: 1}).to.have.any.keys('a', 'b');
     *     expect({a: 1}).to.include.any.keys('a', 'b');
     *
     * A custom error message can be given as the second argument to `expect`.
     *
     *     expect({a: 1}, 'nooo why fail??').to.have.key('b');
     *
     * The alias `.key` can be used interchangeably with `.keys`.
     *
     * @name keys
     * @alias key
     * @param {...String|Array|Object} keys
     * @namespace BDD
     * @api public
     */
  
    function assertKeys (keys) {
      var obj = flag(this, 'object')
        , objType = _.type(obj)
        , keysType = _.type(keys)
        , ssfi = flag(this, 'ssfi')
        , isDeep = flag(this, 'deep')
        , str
        , deepStr = ''
        , actual
        , ok = true
        , flagMsg = flag(this, 'message');
  
      flagMsg = flagMsg ? flagMsg + ': ' : '';
      var mixedArgsMsg = flagMsg + 'when testing keys against an object or an array you must give a single Array|Object|String argument or multiple String arguments';
  
      if (objType === 'Map' || objType === 'Set') {
        deepStr = isDeep ? 'deeply ' : '';
        actual = [];
  
        // Map and Set '.keys' aren't supported in IE 11. Therefore, use .forEach.
        obj.forEach(function (val, key) { actual.push(key) });
  
        if (keysType !== 'Array') {
          keys = Array.prototype.slice.call(arguments);
        }
      } else {
        actual = _.getOwnEnumerableProperties(obj);
  
        switch (keysType) {
          case 'Array':
            if (arguments.length > 1) {
              throw new AssertionError(mixedArgsMsg, undefined, ssfi);
            }
            break;
          case 'Object':
            if (arguments.length > 1) {
              throw new AssertionError(mixedArgsMsg, undefined, ssfi);
            }
            keys = Object.keys(keys);
            break;
          default:
            keys = Array.prototype.slice.call(arguments);
        }
  
        // Only stringify non-Symbols because Symbols would become "Symbol()"
        keys = keys.map(function (val) {
          return typeof val === 'symbol' ? val : String(val);
        });
      }
  
      if (!keys.length) {
        throw new AssertionError(flagMsg + 'keys required', undefined, ssfi);
      }
  
      var len = keys.length
        , any = flag(this, 'any')
        , all = flag(this, 'all')
        , expected = keys;
  
      if (!any && !all) {
        all = true;
      }
  
      // Has any
      if (any) {
        ok = expected.some(function(expectedKey) {
          return actual.some(function(actualKey) {
            if (isDeep) {
              return _.eql(expectedKey, actualKey);
            } else {
              return expectedKey === actualKey;
            }
          });
        });
      }
  
      // Has all
      if (all) {
        ok = expected.every(function(expectedKey) {
          return actual.some(function(actualKey) {
            if (isDeep) {
              return _.eql(expectedKey, actualKey);
            } else {
              return expectedKey === actualKey;
            }
          });
        });
  
        if (!flag(this, 'contains')) {
          ok = ok && keys.length == actual.length;
        }
      }
  
      // Key string
      if (len > 1) {
        keys = keys.map(function(key) {
          return _.inspect(key);
        });
        var last = keys.pop();
        if (all) {
          str = keys.join(', ') + ', and ' + last;
        }
        if (any) {
          str = keys.join(', ') + ', or ' + last;
        }
      } else {
        str = _.inspect(keys[0]);
      }
  
      // Form
      str = (len > 1 ? 'keys ' : 'key ') + str;
  
      // Have / include
      str = (flag(this, 'contains') ? 'contain ' : 'have ') + str;
  
      // Assertion
      this.assert(
          ok
        , 'expected #{this} to ' + deepStr + str
        , 'expected #{this} to not ' + deepStr + str
        , expected.slice(0).sort(_.compareByInspect)
        , actual.sort(_.compareByInspect)
        , true
      );
    }
  
    Assertion.addMethod('keys', assertKeys);
    Assertion.addMethod('key', assertKeys);
  
    /**
     * ### .throw([errorLike], [errMsgMatcher], [msg])
     *
     * When no arguments are provided, `.throw` invokes the target function and
     * asserts that an error is thrown.
     *
     *     var badFn = function () { throw new TypeError('Illegal salmon!'); };
     *
     *     expect(badFn).to.throw();
     *
     * When one argument is provided, and it's an error constructor, `.throw`
     * invokes the target function and asserts that an error is thrown that's an
     * instance of that error constructor.
     *
     *     var badFn = function () { throw new TypeError('Illegal salmon!'); };
     *
     *     expect(badFn).to.throw(TypeError);
     *
     * When one argument is provided, and it's an error instance, `.throw` invokes
     * the target function and asserts that an error is thrown that's strictly
     * (`===`) equal to that error instance.
     *
     *     var err = new TypeError('Illegal salmon!');
     *     var badFn = function () { throw err; };
     *
     *     expect(badFn).to.throw(err);
     *
     * When one argument is provided, and it's a string, `.throw` invokes the
     * target function and asserts that an error is thrown with a message that
     * contains that string.
     *
     *     var badFn = function () { throw new TypeError('Illegal salmon!'); };
     *
     *     expect(badFn).to.throw('salmon');
     *
     * When one argument is provided, and it's a regular expression, `.throw`
     * invokes the target function and asserts that an error is thrown with a
     * message that matches that regular expression.
     *
     *     var badFn = function () { throw new TypeError('Illegal salmon!'); };
     *
     *     expect(badFn).to.throw(/salmon/);
     *
     * When two arguments are provided, and the first is an error instance or
     * constructor, and the second is a string or regular expression, `.throw`
     * invokes the function and asserts that an error is thrown that fulfills both
     * conditions as described above.
     *
     *     var err = new TypeError('Illegal salmon!');
     *     var badFn = function () { throw err; };
     *
     *     expect(badFn).to.throw(TypeError, 'salmon');
     *     expect(badFn).to.throw(TypeError, /salmon/);
     *     expect(badFn).to.throw(err, 'salmon');
     *     expect(badFn).to.throw(err, /salmon/);
     *
     * Add `.not` earlier in the chain to negate `.throw`.
     *
     *     var goodFn = function () {};
     *
     *     expect(goodFn).to.not.throw();
     *
     * However, it's dangerous to negate `.throw` when providing any arguments.
     * The problem is that it creates uncertain expectations by asserting that the
     * target either doesn't throw an error, or that it throws an error but of a
     * different type than the given type, or that it throws an error of the given
     * type but with a message that doesn't include the given string. It's often
     * best to identify the exact output that's expected, and then write an
     * assertion that only accepts that exact output.
     *
     * When the target isn't expected to throw an error, it's often best to assert
     * exactly that.
     *
     *     var goodFn = function () {};
     *
     *     expect(goodFn).to.not.throw(); // Recommended
     *     expect(goodFn).to.not.throw(ReferenceError, 'x'); // Not recommended
     *
     * When the target is expected to throw an error, it's often best to assert
     * that the error is of its expected type, and has a message that includes an
     * expected string, rather than asserting that it doesn't have one of many
     * unexpected types, and doesn't have a message that includes some string.
     *
     *     var badFn = function () { throw new TypeError('Illegal salmon!'); };
     *
     *     expect(badFn).to.throw(TypeError, 'salmon'); // Recommended
     *     expect(badFn).to.not.throw(ReferenceError, 'x'); // Not recommended
     *
     * `.throw` changes the target of any assertions that follow in the chain to
     * be the error object that's thrown.
     *
     *     var err = new TypeError('Illegal salmon!');
     *     err.code = 42;
     *     var badFn = function () { throw err; };
     *
     *     expect(badFn).to.throw(TypeError).with.property('code', 42);
     *
     * `.throw` accepts an optional `msg` argument which is a custom error message
     * to show when the assertion fails. The message can also be given as the
     * second argument to `expect`. When not providing two arguments, always use
     * the second form.
     *
     *     var goodFn = function () {};
     *
     *     expect(goodFn).to.throw(TypeError, 'x', 'nooo why fail??');
     *     expect(goodFn, 'nooo why fail??').to.throw();
     *
     * Due to limitations in ES5, `.throw` may not always work as expected when
     * using a transpiler such as Babel or TypeScript. In particular, it may
     * produce unexpected results when subclassing the built-in `Error` object and
     * then passing the subclassed constructor to `.throw`. See your transpiler's
     * docs for details:
     *
     * - ([Babel](https://babeljs.io/docs/usage/caveats/#classes))
     * - ([TypeScript](https://github.com/Microsoft/TypeScript/wiki/Breaking-Changes#extending-built-ins-like-error-array-and-map-may-no-longer-work))
     *
     * Beware of some common mistakes when using the `throw` assertion. One common
     * mistake is to accidentally invoke the function yourself instead of letting
     * the `throw` assertion invoke the function for you. For example, when
     * testing if a function named `fn` throws, provide `fn` instead of `fn()` as
     * the target for the assertion.
     *
     *     expect(fn).to.throw();     // Good! Tests `fn` as desired
     *     expect(fn()).to.throw();   // Bad! Tests result of `fn()`, not `fn`
     *
     * If you need to assert that your function `fn` throws when passed certain
     * arguments, then wrap a call to `fn` inside of another function.
     *
     *     expect(function () { fn(42); }).to.throw();  // Function expression
     *     expect(() => fn(42)).to.throw();             // ES6 arrow function
     *
     * Another common mistake is to provide an object method (or any stand-alone
     * function that relies on `this`) as the target of the assertion. Doing so is
     * problematic because the `this` context will be lost when the function is
     * invoked by `.throw`; there's no way for it to know what `this` is supposed
     * to be. There are two ways around this problem. One solution is to wrap the
     * method or function call inside of another function. Another solution is to
     * use `bind`.
     *
     *     expect(function () { cat.meow(); }).to.throw();  // Function expression
     *     expect(() => cat.meow()).to.throw();             // ES6 arrow function
     *     expect(cat.meow.bind(cat)).to.throw();           // Bind
     *
     * Finally, it's worth mentioning that it's a best practice in JavaScript to
     * only throw `Error` and derivatives of `Error` such as `ReferenceError`,
     * `TypeError`, and user-defined objects that extend `Error`. No other type of
     * value will generate a stack trace when initialized. With that said, the
     * `throw` assertion does technically support any type of value being thrown,
     * not just `Error` and its derivatives.
     *
     * The aliases `.throws` and `.Throw` can be used interchangeably with
     * `.throw`.
     *
     * @name throw
     * @alias throws
     * @alias Throw
     * @param {Error|ErrorConstructor} errorLike
     * @param {String|RegExp} errMsgMatcher error message
     * @param {String} msg _optional_
     * @see https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Error#Error_types
     * @returns error for chaining (null if no error)
     * @namespace BDD
     * @api public
     */
  
    function assertThrows (errorLike, errMsgMatcher, msg) {
      if (msg) flag(this, 'message', msg);
      var obj = flag(this, 'object')
        , ssfi = flag(this, 'ssfi')
        , flagMsg = flag(this, 'message')
        , negate = flag(this, 'negate') || false;
      new Assertion(obj, flagMsg, ssfi, true).is.a('function');
  
      if (errorLike instanceof RegExp || typeof errorLike === 'string') {
        errMsgMatcher = errorLike;
        errorLike = null;
      }
  
      var caughtErr;
      try {
        obj();
      } catch (err) {
        caughtErr = err;
      }
  
      // If we have the negate flag enabled and at least one valid argument it means we do expect an error
      // but we want it to match a given set of criteria
      var everyArgIsUndefined = errorLike === undefined && errMsgMatcher === undefined;
  
      // If we've got the negate flag enabled and both args, we should only fail if both aren't compatible
      // See Issue #551 and PR #683@GitHub
      var everyArgIsDefined = Boolean(errorLike && errMsgMatcher);
      var errorLikeFail = false;
      var errMsgMatcherFail = false;
  
      // Checking if error was thrown
      if (everyArgIsUndefined || !everyArgIsUndefined && !negate) {
        // We need this to display results correctly according to their types
        var errorLikeString = 'an error';
        if (errorLike instanceof Error) {
          errorLikeString = '#{exp}';
        } else if (errorLike) {
          errorLikeString = _.checkError.getConstructorName(errorLike);
        }
  
        this.assert(
            caughtErr
          , 'expected #{this} to throw ' + errorLikeString
          , 'expected #{this} to not throw an error but #{act} was thrown'
          , errorLike && errorLike.toString()
          , (caughtErr instanceof Error ?
              caughtErr.toString() : (typeof caughtErr === 'string' ? caughtErr : caughtErr &&
                                      _.checkError.getConstructorName(caughtErr)))
        );
      }
  
      if (errorLike && caughtErr) {
        // We should compare instances only if `errorLike` is an instance of `Error`
        if (errorLike instanceof Error) {
          var isCompatibleInstance = _.checkError.compatibleInstance(caughtErr, errorLike);
  
          if (isCompatibleInstance === negate) {
            // These checks were created to ensure we won't fail too soon when we've got both args and a negate
            // See Issue #551 and PR #683@GitHub
            if (everyArgIsDefined && negate) {
              errorLikeFail = true;
            } else {
              this.assert(
                  negate
                , 'expected #{this} to throw #{exp} but #{act} was thrown'
                , 'expected #{this} to not throw #{exp}' + (caughtErr && !negate ? ' but #{act} was thrown' : '')
                , errorLike.toString()
                , caughtErr.toString()
              );
            }
          }
        }
  
        var isCompatibleConstructor = _.checkError.compatibleConstructor(caughtErr, errorLike);
        if (isCompatibleConstructor === negate) {
          if (everyArgIsDefined && negate) {
              errorLikeFail = true;
          } else {
            this.assert(
                negate
              , 'expected #{this} to throw #{exp} but #{act} was thrown'
              , 'expected #{this} to not throw #{exp}' + (caughtErr ? ' but #{act} was thrown' : '')
              , (errorLike instanceof Error ? errorLike.toString() : errorLike && _.checkError.getConstructorName(errorLike))
              , (caughtErr instanceof Error ? caughtErr.toString() : caughtErr && _.checkError.getConstructorName(caughtErr))
            );
          }
        }
      }
  
      if (caughtErr && errMsgMatcher !== undefined && errMsgMatcher !== null) {
        // Here we check compatible messages
        var placeholder = 'including';
        if (errMsgMatcher instanceof RegExp) {
          placeholder = 'matching'
        }
  
        var isCompatibleMessage = _.checkError.compatibleMessage(caughtErr, errMsgMatcher);
        if (isCompatibleMessage === negate) {
          if (everyArgIsDefined && negate) {
              errMsgMatcherFail = true;
          } else {
            this.assert(
              negate
              , 'expected #{this} to throw error ' + placeholder + ' #{exp} but got #{act}'
              , 'expected #{this} to throw error not ' + placeholder + ' #{exp}'
              ,  errMsgMatcher
              ,  _.checkError.getMessage(caughtErr)
            );
          }
        }
      }
  
      // If both assertions failed and both should've matched we throw an error
      if (errorLikeFail && errMsgMatcherFail) {
        this.assert(
          negate
          , 'expected #{this} to throw #{exp} but #{act} was thrown'
          , 'expected #{this} to not throw #{exp}' + (caughtErr ? ' but #{act} was thrown' : '')
          , (errorLike instanceof Error ? errorLike.toString() : errorLike && _.checkError.getConstructorName(errorLike))
          , (caughtErr instanceof Error ? caughtErr.toString() : caughtErr && _.checkError.getConstructorName(caughtErr))
        );
      }
  
      flag(this, 'object', caughtErr);
    };
  
    Assertion.addMethod('throw', assertThrows);
    Assertion.addMethod('throws', assertThrows);
    Assertion.addMethod('Throw', assertThrows);
  
    /**
     * ### .respondTo(method[, msg])
     *
     * When the target is a non-function object, `.respondTo` asserts that the
     * target has a method with the given name `method`. The method can be own or
     * inherited, and it can be enumerable or non-enumerable.
     *
     *     function Cat () {}
     *     Cat.prototype.meow = function () {};
     *
     *     expect(new Cat()).to.respondTo('meow');
     *
     * When the target is a function, `.respondTo` asserts that the target's
     * `prototype` property has a method with the given name `method`. Again, the
     * method can be own or inherited, and it can be enumerable or non-enumerable.
     *
     *     function Cat () {}
     *     Cat.prototype.meow = function () {};
     *
     *     expect(Cat).to.respondTo('meow');
     *
     * Add `.itself` earlier in the chain to force `.respondTo` to treat the
     * target as a non-function object, even if it's a function. Thus, it asserts
     * that the target has a method with the given name `method`, rather than
     * asserting that the target's `prototype` property has a method with the
     * given name `method`.
     *
     *     function Cat () {}
     *     Cat.prototype.meow = function () {};
     *     Cat.hiss = function () {};
     *
     *     expect(Cat).itself.to.respondTo('hiss').but.not.respondTo('meow');
     *
     * When not adding `.itself`, it's important to check the target's type before
     * using `.respondTo`. See the `.a` doc for info on checking a target's type.
     *
     *     function Cat () {}
     *     Cat.prototype.meow = function () {};
     *
     *     expect(new Cat()).to.be.an('object').that.respondsTo('meow');
     *
     * Add `.not` earlier in the chain to negate `.respondTo`.
     *
     *     function Dog () {}
     *     Dog.prototype.bark = function () {};
     *
     *     expect(new Dog()).to.not.respondTo('meow');
     *
     * `.respondTo` accepts an optional `msg` argument which is a custom error
     * message to show when the assertion fails. The message can also be given as
     * the second argument to `expect`.
     *
     *     expect({}).to.respondTo('meow', 'nooo why fail??');
     *     expect({}, 'nooo why fail??').to.respondTo('meow');
     *
     * The alias `.respondsTo` can be used interchangeably with `.respondTo`.
     *
     * @name respondTo
     * @alias respondsTo
     * @param {String} method
     * @param {String} msg _optional_
     * @namespace BDD
     * @api public
     */
  
    function respondTo (method, msg) {
      if (msg) flag(this, 'message', msg);
      var obj = flag(this, 'object')
        , itself = flag(this, 'itself')
        , context = ('function' === typeof obj && !itself)
          ? obj.prototype[method]
          : obj[method];
  
      this.assert(
          'function' === typeof context
        , 'expected #{this} to respond to ' + _.inspect(method)
        , 'expected #{this} to not respond to ' + _.inspect(method)
      );
    }
  
    Assertion.addMethod('respondTo', respondTo);
    Assertion.addMethod('respondsTo', respondTo);
  
    /**
     * ### .itself
     *
     * Forces all `.respondTo` assertions that follow in the chain to behave as if
     * the target is a non-function object, even if it's a function. Thus, it
     * causes `.respondTo` to assert that the target has a method with the given
     * name, rather than asserting that the target's `prototype` property has a
     * method with the given name.
     *
     *     function Cat () {}
     *     Cat.prototype.meow = function () {};
     *     Cat.hiss = function () {};
     *
     *     expect(Cat).itself.to.respondTo('hiss').but.not.respondTo('meow');
     *
     * @name itself
     * @namespace BDD
     * @api public
     */
  
    Assertion.addProperty('itself', function () {
      flag(this, 'itself', true);
    });
  
    /**
     * ### .satisfy(matcher[, msg])
     *
     * Invokes the given `matcher` function with the target being passed as the
     * first argument, and asserts that the value returned is truthy.
     *
     *     expect(1).to.satisfy(function(num) {
     *       return num > 0;
     *     });
     *
     * Add `.not` earlier in the chain to negate `.satisfy`.
     *
     *     expect(1).to.not.satisfy(function(num) {
     *       return num > 2;
     *     });
     *
     * `.satisfy` accepts an optional `msg` argument which is a custom error
     * message to show when the assertion fails. The message can also be given as
     * the second argument to `expect`.
     *
     *     expect(1).to.satisfy(function(num) {
     *       return num > 2;
     *     }, 'nooo why fail??');
     *
     *     expect(1, 'nooo why fail??').to.satisfy(function(num) {
     *       return num > 2;
     *     });
     *
     * The alias `.satisfies` can be used interchangeably with `.satisfy`.
     *
     * @name satisfy
     * @alias satisfies
     * @param {Function} matcher
     * @param {String} msg _optional_
     * @namespace BDD
     * @api public
     */
  
    function satisfy (matcher, msg) {
      if (msg) flag(this, 'message', msg);
      var obj = flag(this, 'object');
      var result = matcher(obj);
      this.assert(
          result
        , 'expected #{this} to satisfy ' + _.objDisplay(matcher)
        , 'expected #{this} to not satisfy' + _.objDisplay(matcher)
        , flag(this, 'negate') ? false : true
        , result
      );
    }
  
    Assertion.addMethod('satisfy', satisfy);
    Assertion.addMethod('satisfies', satisfy);
  
    /**
     * ### .closeTo(expected, delta[, msg])
     *
     * Asserts that the target is a number that's within a given +/- `delta` range
     * of the given number `expected`. However, it's often best to assert that the
     * target is equal to its expected value.
     *
     *     // Recommended
     *     expect(1.5).to.equal(1.5);
     *
     *     // Not recommended
     *     expect(1.5).to.be.closeTo(1, 0.5);
     *     expect(1.5).to.be.closeTo(2, 0.5);
     *     expect(1.5).to.be.closeTo(1, 1);
     *
     * Add `.not` earlier in the chain to negate `.closeTo`.
     *
     *     expect(1.5).to.equal(1.5); // Recommended
     *     expect(1.5).to.not.be.closeTo(3, 1); // Not recommended
     *
     * `.closeTo` accepts an optional `msg` argument which is a custom error
     * message to show when the assertion fails. The message can also be given as
     * the second argument to `expect`.
     *
     *     expect(1.5).to.be.closeTo(3, 1, 'nooo why fail??');
     *     expect(1.5, 'nooo why fail??').to.be.closeTo(3, 1);
     *
     * The alias `.approximately` can be used interchangeably with `.closeTo`.
     *
     * @name closeTo
     * @alias approximately
     * @param {Number} expected
     * @param {Number} delta
     * @param {String} msg _optional_
     * @namespace BDD
     * @api public
     */
  
    function closeTo(expected, delta, msg) {
      if (msg) flag(this, 'message', msg);
      var obj = flag(this, 'object')
        , flagMsg = flag(this, 'message')
        , ssfi = flag(this, 'ssfi');
  
      new Assertion(obj, flagMsg, ssfi, true).is.a('number');
      if (typeof expected !== 'number' || typeof delta !== 'number') {
        flagMsg = flagMsg ? flagMsg + ': ' : '';
        var deltaMessage = delta === undefined ? ", and a delta is required" : "";
        throw new AssertionError(
            flagMsg + 'the arguments to closeTo or approximately must be numbers' + deltaMessage,
            undefined,
            ssfi
        );
      }
  
      this.assert(
          Math.abs(obj - expected) <= delta
        , 'expected #{this} to be close to ' + expected + ' +/- ' + delta
        , 'expected #{this} not to be close to ' + expected + ' +/- ' + delta
      );
    }
  
    Assertion.addMethod('closeTo', closeTo);
    Assertion.addMethod('approximately', closeTo);
  
    // Note: Duplicates are ignored if testing for inclusion instead of sameness.
    function isSubsetOf(subset, superset, cmp, contains, ordered) {
      if (!contains) {
        if (subset.length !== superset.length) return false;
        superset = superset.slice();
      }
  
      return subset.every(function(elem, idx) {
        if (ordered) return cmp ? cmp(elem, superset[idx]) : elem === superset[idx];
  
        if (!cmp) {
          var matchIdx = superset.indexOf(elem);
          if (matchIdx === -1) return false;
  
          // Remove match from superset so not counted twice if duplicate in subset.
          if (!contains) superset.splice(matchIdx, 1);
          return true;
        }
  
        return superset.some(function(elem2, matchIdx) {
          if (!cmp(elem, elem2)) return false;
  
          // Remove match from superset so not counted twice if duplicate in subset.
          if (!contains) superset.splice(matchIdx, 1);
          return true;
        });
      });
    }
  
    /**
     * ### .members(set[, msg])
     *
     * Asserts that the target array has the same members as the given array
     * `set`.
     *
     *     expect([1, 2, 3]).to.have.members([2, 1, 3]);
     *     expect([1, 2, 2]).to.have.members([2, 1, 2]);
     *
     * By default, members are compared using strict (`===`) equality. Add `.deep`
     * earlier in the chain to use deep equality instead. See the `deep-eql`
     * project page for info on the deep equality algorithm:
     * https://github.com/chaijs/deep-eql.
     *
     *     // Target array deeply (but not strictly) has member `{a: 1}`
     *     expect([{a: 1}]).to.have.deep.members([{a: 1}]);
     *     expect([{a: 1}]).to.not.have.members([{a: 1}]);
     *
     * By default, order doesn't matter. Add `.ordered` earlier in the chain to
     * require that members appear in the same order.
     *
     *     expect([1, 2, 3]).to.have.ordered.members([1, 2, 3]);
     *     expect([1, 2, 3]).to.have.members([2, 1, 3])
     *       .but.not.ordered.members([2, 1, 3]);
     *
     * By default, both arrays must be the same size. Add `.include` earlier in
     * the chain to require that the target's members be a superset of the
     * expected members. Note that duplicates are ignored in the subset when
     * `.include` is added.
     *
     *     // Target array is a superset of [1, 2] but not identical
     *     expect([1, 2, 3]).to.include.members([1, 2]);
     *     expect([1, 2, 3]).to.not.have.members([1, 2]);
     *
     *     // Duplicates in the subset are ignored
     *     expect([1, 2, 3]).to.include.members([1, 2, 2, 2]);
     *
     * `.deep`, `.ordered`, and `.include` can all be combined. However, if
     * `.include` and `.ordered` are combined, the ordering begins at the start of
     * both arrays.
     *
     *     expect([{a: 1}, {b: 2}, {c: 3}])
     *       .to.include.deep.ordered.members([{a: 1}, {b: 2}])
     *       .but.not.include.deep.ordered.members([{b: 2}, {c: 3}]);
     *
     * Add `.not` earlier in the chain to negate `.members`. However, it's
     * dangerous to do so. The problem is that it creates uncertain expectations
     * by asserting that the target array doesn't have all of the same members as
     * the given array `set` but may or may not have some of them. It's often best
     * to identify the exact output that's expected, and then write an assertion
     * that only accepts that exact output.
     *
     *     expect([1, 2]).to.not.include(3).and.not.include(4); // Recommended
     *     expect([1, 2]).to.not.have.members([3, 4]); // Not recommended
     *
     * `.members` accepts an optional `msg` argument which is a custom error
     * message to show when the assertion fails. The message can also be given as
     * the second argument to `expect`.
     *
     *     expect([1, 2]).to.have.members([1, 2, 3], 'nooo why fail??');
     *     expect([1, 2], 'nooo why fail??').to.have.members([1, 2, 3]);
     *
     * @name members
     * @param {Array} set
     * @param {String} msg _optional_
     * @namespace BDD
     * @api public
     */
  
    Assertion.addMethod('members', function (subset, msg) {
      if (msg) flag(this, 'message', msg);
      var obj = flag(this, 'object')
        , flagMsg = flag(this, 'message')
        , ssfi = flag(this, 'ssfi');
  
      new Assertion(obj, flagMsg, ssfi, true).to.be.an('array');
      new Assertion(subset, flagMsg, ssfi, true).to.be.an('array');
  
      var contains = flag(this, 'contains');
      var ordered = flag(this, 'ordered');
  
      var subject, failMsg, failNegateMsg;
  
      if (contains) {
        subject = ordered ? 'an ordered superset' : 'a superset';
        failMsg = 'expected #{this} to be ' + subject + ' of #{exp}';
        failNegateMsg = 'expected #{this} to not be ' + subject + ' of #{exp}';
      } else {
        subject = ordered ? 'ordered members' : 'members';
        failMsg = 'expected #{this} to have the same ' + subject + ' as #{exp}';
        failNegateMsg = 'expected #{this} to not have the same ' + subject + ' as #{exp}';
      }
  
      var cmp = flag(this, 'deep') ? _.eql : undefined;
  
      this.assert(
          isSubsetOf(subset, obj, cmp, contains, ordered)
        , failMsg
        , failNegateMsg
        , subset
        , obj
        , true
      );
    });
  
    /**
     * ### .oneOf(list[, msg])
     *
     * Asserts that the target is a member of the given array `list`. However,
     * it's often best to assert that the target is equal to its expected value.
     *
     *     expect(1).to.equal(1); // Recommended
     *     expect(1).to.be.oneOf([1, 2, 3]); // Not recommended
     *
     * Comparisons are performed using strict (`===`) equality.
     *
     * Add `.not` earlier in the chain to negate `.oneOf`.
     *
     *     expect(1).to.equal(1); // Recommended
     *     expect(1).to.not.be.oneOf([2, 3, 4]); // Not recommended
     *
     * It can also be chained with `.contain` or `.include`, which will work with
     * both arrays and strings:
     *
     *     expect('Today is sunny').to.contain.oneOf(['sunny', 'cloudy'])
     *     expect('Today is rainy').to.not.contain.oneOf(['sunny', 'cloudy'])
     *     expect([1,2,3]).to.contain.oneOf([3,4,5])
     *     expect([1,2,3]).to.not.contain.oneOf([4,5,6])
     *
     * `.oneOf` accepts an optional `msg` argument which is a custom error message
     * to show when the assertion fails. The message can also be given as the
     * second argument to `expect`.
     *
     *     expect(1).to.be.oneOf([2, 3, 4], 'nooo why fail??');
     *     expect(1, 'nooo why fail??').to.be.oneOf([2, 3, 4]);
     *
     * @name oneOf
     * @param {Array<*>} list
     * @param {String} msg _optional_
     * @namespace BDD
     * @api public
     */
  
    function oneOf (list, msg) {
      if (msg) flag(this, 'message', msg);
      var expected = flag(this, 'object')
        , flagMsg = flag(this, 'message')
        , ssfi = flag(this, 'ssfi')
        , contains = flag(this, 'contains');
      new Assertion(list, flagMsg, ssfi, true).to.be.an('array');
  
      if (contains) {
        this.assert(
          list.some(function(possibility) { return expected.indexOf(possibility) > -1 })
          , 'expected #{this} to contain one of #{exp}'
          , 'expected #{this} to not contain one of #{exp}'
          , list
          , expected
        );
      } else {
        this.assert(
          list.indexOf(expected) > -1
          , 'expected #{this} to be one of #{exp}'
          , 'expected #{this} to not be one of #{exp}'
          , list
          , expected
        );
      }
    }
  
    Assertion.addMethod('oneOf', oneOf);
  
    /**
     * ### .change(subject[, prop[, msg]])
     *
     * When one argument is provided, `.change` asserts that the given function
     * `subject` returns a different value when it's invoked before the target
     * function compared to when it's invoked afterward. However, it's often best
     * to assert that `subject` is equal to its expected value.
     *
     *     var dots = ''
     *       , addDot = function () { dots += '.'; }
     *       , getDots = function () { return dots; };
     *
     *     // Recommended
     *     expect(getDots()).to.equal('');
     *     addDot();
     *     expect(getDots()).to.equal('.');
     *
     *     // Not recommended
     *     expect(addDot).to.change(getDots);
     *
     * When two arguments are provided, `.change` asserts that the value of the
     * given object `subject`'s `prop` property is different before invoking the
     * target function compared to afterward.
     *
     *     var myObj = {dots: ''}
     *       , addDot = function () { myObj.dots += '.'; };
     *
     *     // Recommended
     *     expect(myObj).to.have.property('dots', '');
     *     addDot();
     *     expect(myObj).to.have.property('dots', '.');
     *
     *     // Not recommended
     *     expect(addDot).to.change(myObj, 'dots');
     *
     * Strict (`===`) equality is used to compare before and after values.
     *
     * Add `.not` earlier in the chain to negate `.change`.
     *
     *     var dots = ''
     *       , noop = function () {}
     *       , getDots = function () { return dots; };
     *
     *     expect(noop).to.not.change(getDots);
     *
     *     var myObj = {dots: ''}
     *       , noop = function () {};
     *
     *     expect(noop).to.not.change(myObj, 'dots');
     *
     * `.change` accepts an optional `msg` argument which is a custom error
     * message to show when the assertion fails. The message can also be given as
     * the second argument to `expect`. When not providing two arguments, always
     * use the second form.
     *
     *     var myObj = {dots: ''}
     *       , addDot = function () { myObj.dots += '.'; };
     *
     *     expect(addDot).to.not.change(myObj, 'dots', 'nooo why fail??');
     *
     *     var dots = ''
     *       , addDot = function () { dots += '.'; }
     *       , getDots = function () { return dots; };
     *
     *     expect(addDot, 'nooo why fail??').to.not.change(getDots);
     *
     * `.change` also causes all `.by` assertions that follow in the chain to
     * assert how much a numeric subject was increased or decreased by. However,
     * it's dangerous to use `.change.by`. The problem is that it creates
     * uncertain expectations by asserting that the subject either increases by
     * the given delta, or that it decreases by the given delta. It's often best
     * to identify the exact output that's expected, and then write an assertion
     * that only accepts that exact output.
     *
     *     var myObj = {val: 1}
     *       , addTwo = function () { myObj.val += 2; }
     *       , subtractTwo = function () { myObj.val -= 2; };
     *
     *     expect(addTwo).to.increase(myObj, 'val').by(2); // Recommended
     *     expect(addTwo).to.change(myObj, 'val').by(2); // Not recommended
     *
     *     expect(subtractTwo).to.decrease(myObj, 'val').by(2); // Recommended
     *     expect(subtractTwo).to.change(myObj, 'val').by(2); // Not recommended
     *
     * The alias `.changes` can be used interchangeably with `.change`.
     *
     * @name change
     * @alias changes
     * @param {String} subject
     * @param {String} prop name _optional_
     * @param {String} msg _optional_
     * @namespace BDD
     * @api public
     */
  
    function assertChanges (subject, prop, msg) {
      if (msg) flag(this, 'message', msg);
      var fn = flag(this, 'object')
        , flagMsg = flag(this, 'message')
        , ssfi = flag(this, 'ssfi');
      new Assertion(fn, flagMsg, ssfi, true).is.a('function');
  
      var initial;
      if (!prop) {
        new Assertion(subject, flagMsg, ssfi, true).is.a('function');
        initial = subject();
      } else {
        new Assertion(subject, flagMsg, ssfi, true).to.have.property(prop);
        initial = subject[prop];
      }
  
      fn();
  
      var final = prop === undefined || prop === null ? subject() : subject[prop];
      var msgObj = prop === undefined || prop === null ? initial : '.' + prop;
  
      // This gets flagged because of the .by(delta) assertion
      flag(this, 'deltaMsgObj', msgObj);
      flag(this, 'initialDeltaValue', initial);
      flag(this, 'finalDeltaValue', final);
      flag(this, 'deltaBehavior', 'change');
      flag(this, 'realDelta', final !== initial);
  
      this.assert(
        initial !== final
        , 'expected ' + msgObj + ' to change'
        , 'expected ' + msgObj + ' to not change'
      );
    }
  
    Assertion.addMethod('change', assertChanges);
    Assertion.addMethod('changes', assertChanges);
  
    /**
     * ### .increase(subject[, prop[, msg]])
     *
     * When one argument is provided, `.increase` asserts that the given function
     * `subject` returns a greater number when it's invoked after invoking the
     * target function compared to when it's invoked beforehand. `.increase` also
     * causes all `.by` assertions that follow in the chain to assert how much
     * greater of a number is returned. It's often best to assert that the return
     * value increased by the expected amount, rather than asserting it increased
     * by any amount.
     *
     *     var val = 1
     *       , addTwo = function () { val += 2; }
     *       , getVal = function () { return val; };
     *
     *     expect(addTwo).to.increase(getVal).by(2); // Recommended
     *     expect(addTwo).to.increase(getVal); // Not recommended
     *
     * When two arguments are provided, `.increase` asserts that the value of the
     * given object `subject`'s `prop` property is greater after invoking the
     * target function compared to beforehand.
     *
     *     var myObj = {val: 1}
     *       , addTwo = function () { myObj.val += 2; };
     *
     *     expect(addTwo).to.increase(myObj, 'val').by(2); // Recommended
     *     expect(addTwo).to.increase(myObj, 'val'); // Not recommended
     *
     * Add `.not` earlier in the chain to negate `.increase`. However, it's
     * dangerous to do so. The problem is that it creates uncertain expectations
     * by asserting that the subject either decreases, or that it stays the same.
     * It's often best to identify the exact output that's expected, and then
     * write an assertion that only accepts that exact output.
     *
     * When the subject is expected to decrease, it's often best to assert that it
     * decreased by the expected amount.
     *
     *     var myObj = {val: 1}
     *       , subtractTwo = function () { myObj.val -= 2; };
     *
     *     expect(subtractTwo).to.decrease(myObj, 'val').by(2); // Recommended
     *     expect(subtractTwo).to.not.increase(myObj, 'val'); // Not recommended
     *
     * When the subject is expected to stay the same, it's often best to assert
     * exactly that.
     *
     *     var myObj = {val: 1}
     *       , noop = function () {};
     *
     *     expect(noop).to.not.change(myObj, 'val'); // Recommended
     *     expect(noop).to.not.increase(myObj, 'val'); // Not recommended
     *
     * `.increase` accepts an optional `msg` argument which is a custom error
     * message to show when the assertion fails. The message can also be given as
     * the second argument to `expect`. When not providing two arguments, always
     * use the second form.
     *
     *     var myObj = {val: 1}
     *       , noop = function () {};
     *
     *     expect(noop).to.increase(myObj, 'val', 'nooo why fail??');
     *
     *     var val = 1
     *       , noop = function () {}
     *       , getVal = function () { return val; };
     *
     *     expect(noop, 'nooo why fail??').to.increase(getVal);
     *
     * The alias `.increases` can be used interchangeably with `.increase`.
     *
     * @name increase
     * @alias increases
     * @param {String|Function} subject
     * @param {String} prop name _optional_
     * @param {String} msg _optional_
     * @namespace BDD
     * @api public
     */
  
    function assertIncreases (subject, prop, msg) {
      if (msg) flag(this, 'message', msg);
      var fn = flag(this, 'object')
        , flagMsg = flag(this, 'message')
        , ssfi = flag(this, 'ssfi');
      new Assertion(fn, flagMsg, ssfi, true).is.a('function');
  
      var initial;
      if (!prop) {
        new Assertion(subject, flagMsg, ssfi, true).is.a('function');
        initial = subject();
      } else {
        new Assertion(subject, flagMsg, ssfi, true).to.have.property(prop);
        initial = subject[prop];
      }
  
      // Make sure that the target is a number
      new Assertion(initial, flagMsg, ssfi, true).is.a('number');
  
      fn();
  
      var final = prop === undefined || prop === null ? subject() : subject[prop];
      var msgObj = prop === undefined || prop === null ? initial : '.' + prop;
  
      flag(this, 'deltaMsgObj', msgObj);
      flag(this, 'initialDeltaValue', initial);
      flag(this, 'finalDeltaValue', final);
      flag(this, 'deltaBehavior', 'increase');
      flag(this, 'realDelta', final - initial);
  
      this.assert(
        final - initial > 0
        , 'expected ' + msgObj + ' to increase'
        , 'expected ' + msgObj + ' to not increase'
      );
    }
  
    Assertion.addMethod('increase', assertIncreases);
    Assertion.addMethod('increases', assertIncreases);
  
    /**
     * ### .decrease(subject[, prop[, msg]])
     *
     * When one argument is provided, `.decrease` asserts that the given function
     * `subject` returns a lesser number when it's invoked after invoking the
     * target function compared to when it's invoked beforehand. `.decrease` also
     * causes all `.by` assertions that follow in the chain to assert how much
     * lesser of a number is returned. It's often best to assert that the return
     * value decreased by the expected amount, rather than asserting it decreased
     * by any amount.
     *
     *     var val = 1
     *       , subtractTwo = function () { val -= 2; }
     *       , getVal = function () { return val; };
     *
     *     expect(subtractTwo).to.decrease(getVal).by(2); // Recommended
     *     expect(subtractTwo).to.decrease(getVal); // Not recommended
     *
     * When two arguments are provided, `.decrease` asserts that the value of the
     * given object `subject`'s `prop` property is lesser after invoking the
     * target function compared to beforehand.
     *
     *     var myObj = {val: 1}
     *       , subtractTwo = function () { myObj.val -= 2; };
     *
     *     expect(subtractTwo).to.decrease(myObj, 'val').by(2); // Recommended
     *     expect(subtractTwo).to.decrease(myObj, 'val'); // Not recommended
     *
     * Add `.not` earlier in the chain to negate `.decrease`. However, it's
     * dangerous to do so. The problem is that it creates uncertain expectations
     * by asserting that the subject either increases, or that it stays the same.
     * It's often best to identify the exact output that's expected, and then
     * write an assertion that only accepts that exact output.
     *
     * When the subject is expected to increase, it's often best to assert that it
     * increased by the expected amount.
     *
     *     var myObj = {val: 1}
     *       , addTwo = function () { myObj.val += 2; };
     *
     *     expect(addTwo).to.increase(myObj, 'val').by(2); // Recommended
     *     expect(addTwo).to.not.decrease(myObj, 'val'); // Not recommended
     *
     * When the subject is expected to stay the same, it's often best to assert
     * exactly that.
     *
     *     var myObj = {val: 1}
     *       , noop = function () {};
     *
     *     expect(noop).to.not.change(myObj, 'val'); // Recommended
     *     expect(noop).to.not.decrease(myObj, 'val'); // Not recommended
     *
     * `.decrease` accepts an optional `msg` argument which is a custom error
     * message to show when the assertion fails. The message can also be given as
     * the second argument to `expect`. When not providing two arguments, always
     * use the second form.
     *
     *     var myObj = {val: 1}
     *       , noop = function () {};
     *
     *     expect(noop).to.decrease(myObj, 'val', 'nooo why fail??');
     *
     *     var val = 1
     *       , noop = function () {}
     *       , getVal = function () { return val; };
     *
     *     expect(noop, 'nooo why fail??').to.decrease(getVal);
     *
     * The alias `.decreases` can be used interchangeably with `.decrease`.
     *
     * @name decrease
     * @alias decreases
     * @param {String|Function} subject
     * @param {String} prop name _optional_
     * @param {String} msg _optional_
     * @namespace BDD
     * @api public
     */
  
    function assertDecreases (subject, prop, msg) {
      if (msg) flag(this, 'message', msg);
      var fn = flag(this, 'object')
        , flagMsg = flag(this, 'message')
        , ssfi = flag(this, 'ssfi');
      new Assertion(fn, flagMsg, ssfi, true).is.a('function');
  
      var initial;
      if (!prop) {
        new Assertion(subject, flagMsg, ssfi, true).is.a('function');
        initial = subject();
      } else {
        new Assertion(subject, flagMsg, ssfi, true).to.have.property(prop);
        initial = subject[prop];
      }
  
      // Make sure that the target is a number
      new Assertion(initial, flagMsg, ssfi, true).is.a('number');
  
      fn();
  
      var final = prop === undefined || prop === null ? subject() : subject[prop];
      var msgObj = prop === undefined || prop === null ? initial : '.' + prop;
  
      flag(this, 'deltaMsgObj', msgObj);
      flag(this, 'initialDeltaValue', initial);
      flag(this, 'finalDeltaValue', final);
      flag(this, 'deltaBehavior', 'decrease');
      flag(this, 'realDelta', initial - final);
  
      this.assert(
        final - initial < 0
        , 'expected ' + msgObj + ' to decrease'
        , 'expected ' + msgObj + ' to not decrease'
      );
    }
  
    Assertion.addMethod('decrease', assertDecreases);
    Assertion.addMethod('decreases', assertDecreases);
  
    /**
     * ### .by(delta[, msg])
     *
     * When following an `.increase` assertion in the chain, `.by` asserts that
     * the subject of the `.increase` assertion increased by the given `delta`.
     *
     *     var myObj = {val: 1}
     *       , addTwo = function () { myObj.val += 2; };
     *
     *     expect(addTwo).to.increase(myObj, 'val').by(2);
     *
     * When following a `.decrease` assertion in the chain, `.by` asserts that the
     * subject of the `.decrease` assertion decreased by the given `delta`.
     *
     *     var myObj = {val: 1}
     *       , subtractTwo = function () { myObj.val -= 2; };
     *
     *     expect(subtractTwo).to.decrease(myObj, 'val').by(2);
     *
     * When following a `.change` assertion in the chain, `.by` asserts that the
     * subject of the `.change` assertion either increased or decreased by the
     * given `delta`. However, it's dangerous to use `.change.by`. The problem is
     * that it creates uncertain expectations. It's often best to identify the
     * exact output that's expected, and then write an assertion that only accepts
     * that exact output.
     *
     *     var myObj = {val: 1}
     *       , addTwo = function () { myObj.val += 2; }
     *       , subtractTwo = function () { myObj.val -= 2; };
     *
     *     expect(addTwo).to.increase(myObj, 'val').by(2); // Recommended
     *     expect(addTwo).to.change(myObj, 'val').by(2); // Not recommended
     *
     *     expect(subtractTwo).to.decrease(myObj, 'val').by(2); // Recommended
     *     expect(subtractTwo).to.change(myObj, 'val').by(2); // Not recommended
     *
     * Add `.not` earlier in the chain to negate `.by`. However, it's often best
     * to assert that the subject changed by its expected delta, rather than
     * asserting that it didn't change by one of countless unexpected deltas.
     *
     *     var myObj = {val: 1}
     *       , addTwo = function () { myObj.val += 2; };
     *
     *     // Recommended
     *     expect(addTwo).to.increase(myObj, 'val').by(2);
     *
     *     // Not recommended
     *     expect(addTwo).to.increase(myObj, 'val').but.not.by(3);
     *
     * `.by` accepts an optional `msg` argument which is a custom error message to
     * show when the assertion fails. The message can also be given as the second
     * argument to `expect`.
     *
     *     var myObj = {val: 1}
     *       , addTwo = function () { myObj.val += 2; };
     *
     *     expect(addTwo).to.increase(myObj, 'val').by(3, 'nooo why fail??');
     *     expect(addTwo, 'nooo why fail??').to.increase(myObj, 'val').by(3);
     *
     * @name by
     * @param {Number} delta
     * @param {String} msg _optional_
     * @namespace BDD
     * @api public
     */
  
    function assertDelta(delta, msg) {
      if (msg) flag(this, 'message', msg);
  
      var msgObj = flag(this, 'deltaMsgObj');
      var initial = flag(this, 'initialDeltaValue');
      var final = flag(this, 'finalDeltaValue');
      var behavior = flag(this, 'deltaBehavior');
      var realDelta = flag(this, 'realDelta');
  
      var expression;
      if (behavior === 'change') {
        expression = Math.abs(final - initial) === Math.abs(delta);
      } else {
        expression = realDelta === Math.abs(delta);
      }
  
      this.assert(
        expression
        , 'expected ' + msgObj + ' to ' + behavior + ' by ' + delta
        , 'expected ' + msgObj + ' to not ' + behavior + ' by ' + delta
      );
    }
  
    Assertion.addMethod('by', assertDelta);
  
    /**
     * ### .extensible
     *
     * Asserts that the target is extensible, which means that new properties can
     * be added to it. Primitives are never extensible.
     *
     *     expect({a: 1}).to.be.extensible;
     *
     * Add `.not` earlier in the chain to negate `.extensible`.
     *
     *     var nonExtensibleObject = Object.preventExtensions({})
     *       , sealedObject = Object.seal({})
     *       , frozenObject = Object.freeze({});
     *
     *     expect(nonExtensibleObject).to.not.be.extensible;
     *     expect(sealedObject).to.not.be.extensible;
     *     expect(frozenObject).to.not.be.extensible;
     *     expect(1).to.not.be.extensible;
     *
     * A custom error message can be given as the second argument to `expect`.
     *
     *     expect(1, 'nooo why fail??').to.be.extensible;
     *
     * @name extensible
     * @namespace BDD
     * @api public
     */
  
    Assertion.addProperty('extensible', function() {
      var obj = flag(this, 'object');
  
      // In ES5, if the argument to this method is a primitive, then it will cause a TypeError.
      // In ES6, a non-object argument will be treated as if it was a non-extensible ordinary object, simply return false.
      // https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/isExtensible
      // The following provides ES6 behavior for ES5 environments.
  
      var isExtensible = obj === Object(obj) && Object.isExtensible(obj);
  
      this.assert(
        isExtensible
        , 'expected #{this} to be extensible'
        , 'expected #{this} to not be extensible'
      );
    });
  
    /**
     * ### .sealed
     *
     * Asserts that the target is sealed, which means that new properties can't be
     * added to it, and its existing properties can't be reconfigured or deleted.
     * However, it's possible that its existing properties can still be reassigned
     * to different values. Primitives are always sealed.
     *
     *     var sealedObject = Object.seal({});
     *     var frozenObject = Object.freeze({});
     *
     *     expect(sealedObject).to.be.sealed;
     *     expect(frozenObject).to.be.sealed;
     *     expect(1).to.be.sealed;
     *
     * Add `.not` earlier in the chain to negate `.sealed`.
     *
     *     expect({a: 1}).to.not.be.sealed;
     *
     * A custom error message can be given as the second argument to `expect`.
     *
     *     expect({a: 1}, 'nooo why fail??').to.be.sealed;
     *
     * @name sealed
     * @namespace BDD
     * @api public
     */
  
    Assertion.addProperty('sealed', function() {
      var obj = flag(this, 'object');
  
      // In ES5, if the argument to this method is a primitive, then it will cause a TypeError.
      // In ES6, a non-object argument will be treated as if it was a sealed ordinary object, simply return true.
      // See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/isSealed
      // The following provides ES6 behavior for ES5 environments.
  
      var isSealed = obj === Object(obj) ? Object.isSealed(obj) : true;
  
      this.assert(
        isSealed
        , 'expected #{this} to be sealed'
        , 'expected #{this} to not be sealed'
      );
    });
  
    /**
     * ### .frozen
     *
     * Asserts that the target is frozen, which means that new properties can't be
     * added to it, and its existing properties can't be reassigned to different
     * values, reconfigured, or deleted. Primitives are always frozen.
     *
     *     var frozenObject = Object.freeze({});
     *
     *     expect(frozenObject).to.be.frozen;
     *     expect(1).to.be.frozen;
     *
     * Add `.not` earlier in the chain to negate `.frozen`.
     *
     *     expect({a: 1}).to.not.be.frozen;
     *
     * A custom error message can be given as the second argument to `expect`.
     *
     *     expect({a: 1}, 'nooo why fail??').to.be.frozen;
     *
     * @name frozen
     * @namespace BDD
     * @api public
     */
  
    Assertion.addProperty('frozen', function() {
      var obj = flag(this, 'object');
  
      // In ES5, if the argument to this method is a primitive, then it will cause a TypeError.
      // In ES6, a non-object argument will be treated as if it was a frozen ordinary object, simply return true.
      // See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/isFrozen
      // The following provides ES6 behavior for ES5 environments.
  
      var isFrozen = obj === Object(obj) ? Object.isFrozen(obj) : true;
  
      this.assert(
        isFrozen
        , 'expected #{this} to be frozen'
        , 'expected #{this} to not be frozen'
      );
    });
  
    /**
     * ### .finite
     *
     * Asserts that the target is a number, and isn't `NaN` or positive/negative
     * `Infinity`.
     *
     *     expect(1).to.be.finite;
     *
     * Add `.not` earlier in the chain to negate `.finite`. However, it's
     * dangerous to do so. The problem is that it creates uncertain expectations
     * by asserting that the subject either isn't a number, or that it's `NaN`, or
     * that it's positive `Infinity`, or that it's negative `Infinity`. It's often
     * best to identify the exact output that's expected, and then write an
     * assertion that only accepts that exact output.
     *
     * When the target isn't expected to be a number, it's often best to assert
     * that it's the expected type, rather than asserting that it isn't one of
     * many unexpected types.
     *
     *     expect('foo').to.be.a('string'); // Recommended
     *     expect('foo').to.not.be.finite; // Not recommended
     *
     * When the target is expected to be `NaN`, it's often best to assert exactly
     * that.
     *
     *     expect(NaN).to.be.NaN; // Recommended
     *     expect(NaN).to.not.be.finite; // Not recommended
     *
     * When the target is expected to be positive infinity, it's often best to
     * assert exactly that.
     *
     *     expect(Infinity).to.equal(Infinity); // Recommended
     *     expect(Infinity).to.not.be.finite; // Not recommended
     *
     * When the target is expected to be negative infinity, it's often best to
     * assert exactly that.
     *
     *     expect(-Infinity).to.equal(-Infinity); // Recommended
     *     expect(-Infinity).to.not.be.finite; // Not recommended
     *
     * A custom error message can be given as the second argument to `expect`.
     *
     *     expect('foo', 'nooo why fail??').to.be.finite;
     *
     * @name finite
     * @namespace BDD
     * @api public
     */
  
    Assertion.addProperty('finite', function(msg) {
      var obj = flag(this, 'object');
  
      this.assert(
          typeof obj === 'number' && isFinite(obj)
        , 'expected #{this} to be a finite number'
        , 'expected #{this} to not be a finite number'
      );
    });
  };
  