text: var less, tree;
  
  // Node.js does not have a header file added which defines less
  if (less === undefined) {
      less = exports;
      tree = require('./tree');
      less.mode = 'node';
  }
  //
  // less.js - parser
  //
  //    A relatively straight-forward predictive parser.
  //    There is no tokenization/lexing stage, the input is parsed
  //    in one sweep.
  //
  //    To make the parser fast enough to run in the browser, several
  //    optimization had to be made:
  //
  //    - Matching and slicing on a huge input is often cause of slowdowns.
  //      The solution is to chunkify the input into smaller strings.
  //      The chunks are stored in the `chunks` var,
  //      `j` holds the current chunk index, and `currentPos` holds
  //      the index of the current chunk in relation to `input`.
  //      This gives us an almost 4x speed-up.
  //
  //    - In many cases, we don't need to match individual tokens;
  //      for example, if a value doesn't hold any variables, operations
  //      or dynamic references, the parser can effectively 'skip' it,
  //      treating it as a literal.
  //      An example would be '1px solid #000' - which evaluates to itself,
  //      we don't need to know what the individual components are.
  //      The drawback, of course is that you don't get the benefits of
  //      syntax-checking on the CSS. This gives us a 50% speed-up in the parser,
  //      and a smaller speed-up in the code-gen.
  //
  //
  //    Token matching is done with the `$` function, which either takes
  //    a terminal string or regexp, or a non-terminal function to call.
  //    It also takes care of moving all the indices forwards.
  //
  //
  less.Parser = function Parser(env) {
      var input,       // LeSS input string
          i,           // current index in `input`
          j,           // current chunk
          saveStack = [],   // holds state for backtracking
          furthest,    // furthest index the parser has gone to
          chunks,      // chunkified input
          current,     // current chunk
          currentPos,  // index of current chunk, in `input`
          parser,
          parsers,
          rootFilename = env && env.filename;
  
      // Top parser on an import tree must be sure there is one "env"
      // which will then be passed around by reference.
      if (!(env instanceof tree.parseEnv)) {
          env = new tree.parseEnv(env);
      }
  
      var imports = this.imports = {
          paths: env.paths || [],  // Search paths, when importing
          queue: [],               // Files which haven't been imported yet
          files: env.files,        // Holds the imported parse trees
          contents: env.contents,  // Holds the imported file contents
          contentsIgnoredChars: env.contentsIgnoredChars, // lines inserted, not in the original less
          mime:  env.mime,         // MIME type of .less files
          error: null,             // Error in parsing/evaluating an import
          push: function (path, currentFileInfo, importOptions, callback) {
              var parserImports = this;
              this.queue.push(path);
  
              var fileParsedFunc = function (e, root, fullPath) {
                  parserImports.queue.splice(parserImports.queue.indexOf(path), 1); // Remove the path from the queue
  
                  var importedPreviously = fullPath in parserImports.files || fullPath === rootFilename;
  
                  parserImports.files[fullPath] = root;                        // Store the root
  
                  if (e && !parserImports.error) { parserImports.error = e; }
  
                  callback(e, root, importedPreviously, fullPath);
              };
  
              if (less.Parser.importer) {
                  less.Parser.importer(path, currentFileInfo, fileParsedFunc, env);
              } else {
                  less.Parser.fileLoader(path, currentFileInfo, function(e, contents, fullPath, newFileInfo) {
                      if (e) {fileParsedFunc(e); return;}
  
                      var newEnv = new tree.parseEnv(env);
  
                      newEnv.currentFileInfo = newFileInfo;
                      newEnv.processImports = false;
                      newEnv.contents[fullPath] = contents;
  
                      if (currentFileInfo.reference || importOptions.reference) {
                          newFileInfo.reference = true;
                      }
  
                      if (importOptions.inline) {
                          fileParsedFunc(null, contents, fullPath);
                      } else {
                          new(less.Parser)(newEnv).parse(contents, function (e, root) {
                              fileParsedFunc(e, root, fullPath);
                          });
                      }
                  }, env);
              }
          }
      };
  
      function save()    { currentPos = i; saveStack.push( { current: current, i: i, j: j }); }
      function restore() { var state = saveStack.pop(); current = state.current; currentPos = i = state.i; j = state.j; }
      function forget() { saveStack.pop(); }
  
      function sync() {
          if (i > currentPos) {
              current = current.slice(i - currentPos);
              currentPos = i;
          }
      }
      function isWhitespace(str, pos) {
          var code = str.charCodeAt(pos | 0);
          return (code <= 32) && (code === 32 || code === 10 || code === 9);
      }
      //
      // Parse from a token, regexp or string, and move forward if match
      //
      function $(tok) {
          var tokType = typeof tok,
              match, length;
  
          // Either match a single character in the input,
          // or match a regexp in the current chunk (`current`).
          //
          if (tokType === "string") {
              if (input.charAt(i) !== tok) {
                  return null;
              }
              skipWhitespace(1);
              return tok;
          }
  
          // regexp
          sync ();
          if (! (match = tok.exec(current))) {
              return null;
          }
  
          length = match[0].length;
  
          // The match is confirmed, add the match length to `i`,
          // and consume any extra white-space characters (' ' || '\n')
          // which come after that. The reason for this is that LeSS's
          // grammar is mostly white-space insensitive.
          //
          skipWhitespace(length);
  
          if(typeof(match) === 'string') {
              return match;
          } else {
              return match.length === 1 ? match[0] : match;
          }
      }
  
      // Specialization of $(tok)
      function $re(tok) {
          if (i > currentPos) {
              current = current.slice(i - currentPos);
              currentPos = i;
          }
          var m = tok.exec(current);
          if (!m) {
              return null;
          }
  
          skipWhitespace(m[0].length);
          if(typeof m === "string") {
              return m;
          }
  
          return m.length === 1 ? m[0] : m;
      }
  
      var _$re = $re;
  
      // Specialization of $(tok)
      function $char(tok) {
          if (input.charAt(i) !== tok) {
              return null;
          }
          skipWhitespace(1);
          return tok;
      }
  
      function skipWhitespace(length) {
          var oldi = i, oldj = j,
              curr = i - currentPos,
              endIndex = i + current.length - curr,
              mem = (i += length),
              inp = input,
              c;
  
          for (; i < endIndex; i++) {
              c = inp.charCodeAt(i);
              if (c > 32) {
                  break;
              }
  
              if ((c !== 32) && (c !== 10) && (c !== 9) && (c !== 13)) {
                  break;
              }
           }
  
          current = current.slice(length + i - mem + curr);
          currentPos = i;
  
          if (!current.length && (j < chunks.length - 1)) {
              current = chunks[++j];
              skipWhitespace(0); // skip space at the beginning of a chunk
              return true; // things changed
          }
  
          return oldi !== i || oldj !== j;
      }
  
      function expect(arg, msg) {
          // some older browsers return typeof 'function' for RegExp
          var result = (Object.prototype.toString.call(arg) === '[object Function]') ? arg.call(parsers) : $(arg);
          if (result) {
              return result;
          }
          error(msg || (typeof(arg) === 'string' ? "expected '" + arg + "' got '" + input.charAt(i) + "'"
                                                 : "unexpected token"));
      }
  
      // Specialization of expect()
      function expectChar(arg, msg) {
          if (input.charAt(i) === arg) {
              skipWhitespace(1);
              return arg;
          }
          error(msg || "expected '" + arg + "' got '" + input.charAt(i) + "'");
      }
  
      function error(msg, type) {
          var e = new Error(msg);
          e.index = i;
          e.type = type || 'Syntax';
          throw e;
      }
  
      // Same as $(), but don't change the state of the parser,
      // just return the match.
      function peek(tok) {
          if (typeof(tok) === 'string') {
              return input.charAt(i) === tok;
          } else {
              return tok.test(current);
          }
      }
  
      // Specialization of peek()
      function peekChar(tok) {
          return input.charAt(i) === tok;
      }
  
  
      function getInput(e, env) {
          if (e.filename && env.currentFileInfo.filename && (e.filename !== env.currentFileInfo.filename)) {
              return parser.imports.contents[e.filename];
          } else {
              return input;
          }
      }
  
      function getLocation(index, inputStream) {
          var n = index + 1,
              line = null,
              column = -1;
  
          while (--n >= 0 && inputStream.charAt(n) !== '\n') {
              column++;
          }
  
          if (typeof index === 'number') {
              line = (inputStream.slice(0, index).match(/\n/g) || "").length;
          }
  
          return {
              line: line,
              column: column
          };
      }
  
      function getDebugInfo(index, inputStream, env) {
          var filename = env.currentFileInfo.filename;
          if(less.mode !== 'browser' && less.mode !== 'rhino') {
              filename = require('path').resolve(filename);
          }
  
          return {
              lineNumber: getLocation(index, inputStream).line + 1,
              fileName: filename
          };
      }
  
      function LessError(e, env) {
          var input = getInput(e, env),
              loc = getLocation(e.index, input),
              line = loc.line,
              col  = loc.column,
              callLine = e.call && getLocation(e.call, input).line,
              lines = input.split('\n');
  
          this.type = e.type || 'Syntax';
          this.message = e.message;
          this.filename = e.filename || env.currentFileInfo.filename;
          this.index = e.index;
          this.line = typeof(line) === 'number' ? line + 1 : null;
          this.callLine = callLine + 1;
          this.callExtract = lines[callLine];
          this.stack = e.stack;
          this.column = col;
          this.extract = [
              lines[line - 1],
              lines[line],
              lines[line + 1]
          ];
      }
  
      LessError.prototype = new Error();
      LessError.prototype.constructor = LessError;
  
      this.env = env = env || {};
  
      // The optimization level dictates the thoroughness of the parser,
      // the lower the number, the less nodes it will create in the tree.
      // This could matter for debugging, or if you want to access
      // the individual nodes in the tree.
      this.optimization = ('optimization' in this.env) ? this.env.optimization : 1;
  
      //
      // The Parser
      //
      parser = {
  
          imports: imports,
          //
          // Parse an input string into an abstract syntax tree,
          // @param str A string containing 'less' markup
          // @param callback call `callback` when done.
          // @param [additionalData] An optional map which can contains vars - a map (key, value) of variables to apply
          //
          parse: function (str, callback, additionalData) {
              var root, line, lines, error = null, globalVars, modifyVars, preText = "";
  
              i = j = currentPos = furthest = 0;
  
              globalVars = (additionalData && additionalData.globalVars) ? less.Parser.serializeVars(additionalData.globalVars) + '\n' : '';
              modifyVars = (additionalData && additionalData.modifyVars) ? '\n' + less.Parser.serializeVars(additionalData.modifyVars) : '';
  
              if (globalVars || (additionalData && additionalData.banner)) {
                  preText = ((additionalData && additionalData.banner) ? additionalData.banner : "") + globalVars;
                  parser.imports.contentsIgnoredChars[env.currentFileInfo.filename] = preText.length;
              }
  
              str = str.replace(/\r\n/g, '\n');
              // Remove potential UTF Byte Order Mark
              input = str = preText + str.replace(/^\uFEFF/, '') + modifyVars;
              parser.imports.contents[env.currentFileInfo.filename] = str;
  
              // Split the input into chunks.
              chunks = (function (input) {
                  var len = input.length, level = 0, parenLevel = 0,
                      lastOpening, lastOpeningParen, lastMultiComment, lastMultiCommentEndBrace,
                      chunks = [], emitFrom = 0,
                      parserCurrentIndex, currentChunkStartIndex, cc, cc2, matched;
  
                  function fail(msg, index) {
                      error = new(LessError)({
                          index: index || parserCurrentIndex,
                          type: 'Parse',
                          message: msg,
                          filename: env.currentFileInfo.filename
                      }, env);
                  }
  
                  function emitChunk(force) {
                      var len = parserCurrentIndex - emitFrom;
                      if (((len < 512) && !force) || !len) {
                          return;
                      }
                      chunks.push(input.slice(emitFrom, parserCurrentIndex + 1));
                      emitFrom = parserCurrentIndex + 1;
                  }
  
                  for (parserCurrentIndex = 0; parserCurrentIndex < len; parserCurrentIndex++) {
                      cc = input.charCodeAt(parserCurrentIndex);
                      if (((cc >= 97) && (cc <= 122)) || (cc < 34)) {
                          // a-z or whitespace
                          continue;
                      }
  
                      switch (cc) {
                          case 40:                        // (
                              parenLevel++; 
                              lastOpeningParen = parserCurrentIndex; 
                              continue;
                          case 41:                        // )
                              if (--parenLevel < 0) {
                                  return fail("missing opening `(`");
                              }
                              continue;
                          case 59:                        // ;
                              if (!parenLevel) { emitChunk(); }
                              continue;
                          case 123:                       // {
                              level++; 
                              lastOpening = parserCurrentIndex; 
                              continue;
                          case 125:                       // }
                              if (--level < 0) {
                                  return fail("missing opening `{`");
                              }
                              if (!level && !parenLevel) { emitChunk(); }
                              continue;
                          case 92:                        // \
                              if (parserCurrentIndex < len - 1) { parserCurrentIndex++; continue; }
                              return fail("unescaped `\\`");
                          case 34:
                          case 39:
                          case 96:                        // ", ' and `
                              matched = 0;
                              currentChunkStartIndex = parserCurrentIndex;
                              for (parserCurrentIndex = parserCurrentIndex + 1; parserCurrentIndex < len; parserCurrentIndex++) {
                                  cc2 = input.charCodeAt(parserCurrentIndex);
                                  if (cc2 > 96) { continue; }
                                  if (cc2 == cc) { matched = 1; break; }
                                  if (cc2 == 92) {        // \
                                      if (parserCurrentIndex == len - 1) {
                                          return fail("unescaped `\\`");
                                      }
                                      parserCurrentIndex++;
                                  }
                              }
                              if (matched) { continue; }
                              return fail("unmatched `" + String.fromCharCode(cc) + "`", currentChunkStartIndex);
                          case 47:                        // /, check for comment
                              if (parenLevel || (parserCurrentIndex == len - 1)) { continue; }
                              cc2 = input.charCodeAt(parserCurrentIndex + 1);
                              if (cc2 == 47) {
                                  // //, find lnfeed
                                  for (parserCurrentIndex = parserCurrentIndex + 2; parserCurrentIndex < len; parserCurrentIndex++) {
                                      cc2 = input.charCodeAt(parserCurrentIndex);
                                      if ((cc2 <= 13) && ((cc2 == 10) || (cc2 == 13))) { break; }
                                  }
                              } else if (cc2 == 42) {
                                  // /*, find */
                                  lastMultiComment = currentChunkStartIndex = parserCurrentIndex;
                                  for (parserCurrentIndex = parserCurrentIndex + 2; parserCurrentIndex < len - 1; parserCurrentIndex++) {
                                      cc2 = input.charCodeAt(parserCurrentIndex);
                                      if (cc2 == 125) { lastMultiCommentEndBrace = parserCurrentIndex; }
                                      if (cc2 != 42) { continue; }
                                      if (input.charCodeAt(parserCurrentIndex + 1) == 47) { break; }
                                  }
                                  if (parserCurrentIndex == len - 1) {
                                      return fail("missing closing `*/`", currentChunkStartIndex);
                                  }
                                  parserCurrentIndex++;
                              }
                              continue;
                          case 42:                       // *, check for unmatched */
                              if ((parserCurrentIndex < len - 1) && (input.charCodeAt(parserCurrentIndex + 1) == 47)) {
                                  return fail("unmatched `/*`");
                              }
                              continue;
                      }
                  }
  
                  if (level !== 0) {
                      if ((lastMultiComment > lastOpening) && (lastMultiCommentEndBrace > lastMultiComment)) {
                          return fail("missing closing `}` or `*/`", lastOpening);
                      } else {
                          return fail("missing closing `}`", lastOpening);
                      }
                  } else if (parenLevel !== 0) {
                      return fail("missing closing `)`", lastOpeningParen);
                  }
  
                  emitChunk(true);
                  return chunks;
              })(str);
  
              if (error) {
                  return callback(new(LessError)(error, env));
              }
  
              current = chunks[0];
  
              // Start with the primary rule.
              // The whole syntax tree is held under a Ruleset node,
              // with the `root` property set to true, so no `{}` are
              // output. The callback is called when the input is parsed.
              try {
                  root = new(tree.Ruleset)(null, this.parsers.primary());
                  root.root = true;
                  root.firstRoot = true;
              } catch (e) {
                  return callback(new(LessError)(e, env));
              }
  
              root.toCSS = (function (evaluate) {
                  return function (options, variables) {
                      options = options || {};
                      var evaldRoot,
                          css,
                          evalEnv = new tree.evalEnv(options);
                          
                      //
                      // Allows setting variables with a hash, so:
                      //
                      //   `{ color: new(tree.Color)('#f01') }` will become:
                      //
                      //   new(tree.Rule)('@color',
                      //     new(tree.Value)([
                      //       new(tree.Expression)([
                      //         new(tree.Color)('#f01')
                      //       ])
                      //     ])
                      //   )
                      //
                      if (typeof(variables) === 'object' && !Array.isArray(variables)) {
                          variables = Object.keys(variables).map(function (k) {
                              var value = variables[k];
  
                              if (! (value instanceof tree.Value)) {
                                  if (! (value instanceof tree.Expression)) {
                                      value = new(tree.Expression)([value]);
                                  }
                                  value = new(tree.Value)([value]);
                              }
                              return new(tree.Rule)('@' + k, value, false, null, 0);
                          });
                          evalEnv.frames = [new(tree.Ruleset)(null, variables)];
                      }
  
                      try {
                          var preEvalVisitors = [],
                              visitors = [
                                  new(tree.joinSelectorVisitor)(),
                                  new(tree.processExtendsVisitor)(),
                                  new(tree.toCSSVisitor)({compress: Boolean(options.compress)})
                              ], i, root = this;
  
                          if (options.plugins) {
                              for(i =0; i < options.plugins.length; i++) {
                                  if (options.plugins[i].isPreEvalVisitor) {
                                      preEvalVisitors.push(options.plugins[i]);
                                  } else {
                                      if (options.plugins[i].isPreVisitor) {
                                          visitors.splice(0, 0, options.plugins[i]);
                                      } else {
                                          visitors.push(options.plugins[i]);
                                      }
                                  }
                              }
                          }
  
                          for(i = 0; i < preEvalVisitors.length; i++) {
                              preEvalVisitors[i].run(root);
                          }
  
                          evaldRoot = evaluate.call(root, evalEnv);
  
                          for(i = 0; i < visitors.length; i++) {
                              visitors[i].run(evaldRoot);
                          }
  
                          if (options.sourceMap) {
                              evaldRoot = new tree.sourceMapOutput(
                                  {
                                      contentsIgnoredCharsMap: parser.imports.contentsIgnoredChars,
                                      writeSourceMap: options.writeSourceMap,
                                      rootNode: evaldRoot,
                                      contentsMap: parser.imports.contents,
                                      sourceMapFilename: options.sourceMapFilename,
                                      sourceMapURL: options.sourceMapURL,
                                      outputFilename: options.sourceMapOutputFilename,
                                      sourceMapBasepath: options.sourceMapBasepath,
                                      sourceMapRootpath: options.sourceMapRootpath,
                                      outputSourceFiles: options.outputSourceFiles,
                                      sourceMapGenerator: options.sourceMapGenerator
                                  });
                          }
  
                          css = evaldRoot.toCSS({
                                  compress: Boolean(options.compress),
                                  dumpLineNumbers: env.dumpLineNumbers,
                                  strictUnits: Boolean(options.strictUnits),
                                  numPrecision: 8});
                      } catch (e) {
                          throw new(LessError)(e, env);
                      }
  
                      if (options.cleancss && less.mode === 'node') {
                          var CleanCSS = require('clean-css'),
                              cleancssOptions = options.cleancssOptions || {};
  
                          if (cleancssOptions.keepSpecialComments === undefined) {
                              cleancssOptions.keepSpecialComments = "*";
                          }
                          cleancssOptions.processImport = false;
                          cleancssOptions.noRebase = true;
                          if (cleancssOptions.noAdvanced === undefined) {
                              cleancssOptions.noAdvanced = true;
                          }
  
                          return new CleanCSS(cleancssOptions).minify(css);
                      } else if (options.compress) {
                          return css.replace(/(^(\s)+)|((\s)+$)/g, "");
                      } else {
                          return css;
                      }
                  };
              })(root.eval);
  
              // If `i` is smaller than the `input.length - 1`,
              // it means the parser wasn't able to parse the whole
              // string, so we've got a parsing error.
              //
              // We try to extract a \n delimited string,
              // showing the line where the parse error occured.
              // We split it up into two parts (the part which parsed,
              // and the part which didn't), so we can color them differently.
              if (i < input.length - 1) {
                  i = furthest;
                  var loc = getLocation(i, input);
                  lines = input.split('\n');
                  line = loc.line + 1;
  
                  error = {
                      type: "Parse",
                      message: "Unrecognised input",
                      index: i,
                      filename: env.currentFileInfo.filename,
                      line: line,
                      column: loc.column,
                      extract: [
                          lines[line - 2],
                          lines[line - 1],
                          lines[line]
                      ]
                  };
              }
  
              var finish = function (e) {
                  e = error || e || parser.imports.error;
  
                  if (e) {
                      if (!(e instanceof LessError)) {
                          e = new(LessError)(e, env);
                      }
  
                      return callback(e);
                  }
                  else {
                      return callback(null, root);
                  }
              };
  
              if (env.processImports !== false) {
                  new tree.importVisitor(this.imports, finish)
                      .run(root);
              } else {
                  return finish();
              }
          },
  
          //
          // Here in, the parsing rules/functions
          //
          // The basic structure of the syntax tree generated is as follows:
          //
          //   Ruleset ->  Rule -> Value -> Expression -> Entity
          //
          // Here's some LESS code:
          //
          //    .class {
          //      color: #fff;
          //      border: 1px solid #000;
          //      width: @w + 4px;
          //      > .child {...}
          //    }
          //
          // And here's what the parse tree might look like:
          //
          //     Ruleset (Selector '.class', [
          //         Rule ("color",  Value ([Expression [Color #fff]]))
          //         Rule ("border", Value ([Expression [Dimension 1px][Keyword "solid"][Color #000]]))
          //         Rule ("width",  Value ([Expression [Operation "+" [Variable "@w"][Dimension 4px]]]))
          //         Ruleset (Selector [Element '>', '.child'], [...])
          //     ])
          //
          //  In general, most rules will try to parse a token with the `$()` function, and if the return
          //  value is truly, will return a new node, of the relevant type. Sometimes, we need to check
          //  first, before parsing, that's when we use `peek()`.
          //
          parsers: parsers = {
              //
              // The `primary` rule is the *entry* and *exit* point of the parser.
              // The rules here can appear at any level of the parse tree.
              //
              // The recursive nature of the grammar is an interplay between the `block`
              // rule, which represents `{ ... }`, the `ruleset` rule, and this `primary` rule,
              // as represented by this simplified grammar:
              //
              //     primary  →  (ruleset | rule)+
              //     ruleset  →  selector+ block
              //     block    →  '{' primary '}'
              //
              // Only at one point is the primary rule not called from the
              // block rule: at the root level.
              //
              primary: function () {
                  var mixin = this.mixin, $re = _$re, root = [], node;
  
                  while (current)
                  {
                      node = this.extendRule() || mixin.definition() || this.rule() || this.ruleset() ||
                          mixin.call() || this.comment() || this.rulesetCall() || this.directive();
                      if (node) {
                          root.push(node);
                      } else {
                          if (!($re(/^[\s\n]+/) || $re(/^;+/))) {
                              break;
                          }
                      }
                      if (peekChar('}')) {
                          break;
                      }
                  }
  
                  return root;
              },
  
              // We create a Comment node for CSS comments `/* */`,
              // but keep the LeSS comments `//` silent, by just skipping
              // over them.
              comment: function () {
                  var comment;
  
                  if (input.charAt(i) !== '/') { return; }
  
                  if (input.charAt(i + 1) === '/') {
                      return new(tree.Comment)($re(/^\/\/.*/), true, i, env.currentFileInfo);
                  }
                  comment = $re(/^\/\*(?:[^*]|\*+[^\/*])*\*+\/\n?/);
                  if (comment) {
                      return new(tree.Comment)(comment, false, i, env.currentFileInfo);
                  }
              },
  
              comments: function () {
                  var comment, comments = [];
  
                  while(true) {
                      comment = this.comment();
                      if (!comment) {
                          break;
                      }
                      comments.push(comment);
                  }
  
                  return comments;
              },
  
              //
              // Entities are tokens which can be found inside an Expression
              //
              entities: {
                  //
                  // A string, which supports escaping " and '
                  //
                  //     "milky way" 'he\'s the one!'
                  //
                  quoted: function () {
                      var str, j = i, e, index = i;
  
                      if (input.charAt(j) === '~') { j++; e = true; } // Escaped strings
                      if (input.charAt(j) !== '"' && input.charAt(j) !== "'") { return; }
  
                      if (e) { $char('~'); }
  
                      str = $re(/^"((?:[^"\\\r\n]|\\.)*)"|'((?:[^'\\\r\n]|\\.)*)'/);
                      if (str) {
                          return new(tree.Quoted)(str[0], str[1] || str[2], e, index, env.currentFileInfo);
                      }
                  },
  
                  //
                  // A catch-all word, such as:
                  //
                  //     black border-collapse
                  //
                  keyword: function () {
                      var k;
  
                      k = $re(/^[_A-Za-z-][_A-Za-z0-9-]*/);
                      if (k) {
                          var color = tree.Color.fromKeyword(k);
                          if (color) {
                              return color;
                          }
                          return new(tree.Keyword)(k);
                      }
                  },
  
                  //
                  // A function call
                  //
                  //     rgb(255, 0, 255)
                  //
                  // We also try to catch IE's `alpha()`, but let the `alpha` parser
                  // deal with the details.
                  //
                  // The arguments are parsed with the `entities.arguments` parser.
                  //
                  call: function () {
                      var name, nameLC, args, alpha_ret, index = i;
  
                      name = /^([\w-]+|%|progid:[\w\.]+)\(/.exec(current);
                      if (!name) { return; }
  
                      name = name[1];
                      nameLC = name.toLowerCase();
                      if (nameLC === 'url') {
                          return null;
                      }
  
                      i += name.length;
  
                      if (nameLC === 'alpha') {
                          alpha_ret = parsers.alpha();
                          if(typeof alpha_ret !== 'undefined') {
                              return alpha_ret;
                          }
                      }
  
                      $char('('); // Parse the '(' and consume whitespace.
  
                      args = this.arguments();
  
                      if (! $char(')')) {
                          return;
                      }
  
                      if (name) { return new(tree.Call)(name, args, index, env.currentFileInfo); }
                  },
                  arguments: function () {
                      var args = [], arg;
  
                      while (true) {
                          arg = this.assignment() || parsers.expression();
                          if (!arg) {
                              break;
                          }
                          args.push(arg);
                          if (! $char(',')) {
                              break;
                          }
                      }
                      return args;
                  },
                  literal: function () {
                      return this.dimension() ||
                             this.color() ||
                             this.quoted() ||
                             this.unicodeDescriptor();
                  },
  
                  // Assignments are argument entities for calls.
                  // They are present in ie filter properties as shown below.
                  //
                  //     filter: progid:DXImageTransform.Microsoft.Alpha( *opacity=50* )
                  //
  
                  assignment: function () {
                      var key, value;
                      key = $re(/^\w+(?=\s?=)/i);
                      if (!key) {
                          return;
                      }
                      if (!$char('=')) {
                          return;
                      }
                      value = parsers.entity();
                      if (value) {
                          return new(tree.Assignment)(key, value);
                      }
                  },
  
                  //
                  // Parse url() tokens
                  //
                  // We use a specific rule for urls, because they don't really behave like
                  // standard function calls. The difference is that the argument doesn't have
                  // to be enclosed within a string, so it can't be parsed as an Expression.
                  //
                  url: function () {
                      var value;
  
                      if (input.charAt(i) !== 'u' || !$re(/^url\(/)) {
                          return;
                      }
  
                      value = this.quoted() || this.variable() ||
                              $re(/^(?:(?:\\[\(\)'"])|[^\(\)'"])+/) || "";
  
                      expectChar(')');
  
                      return new(tree.URL)((value.value != null || value instanceof tree.Variable)
                                          ? value : new(tree.Anonymous)(value), env.currentFileInfo);
                  },
  
                  //
                  // A Variable entity, such as `@fink`, in
                  //
                  //     width: @fink + 2px
                  //
                  // We use a different parser for variable definitions,
                  // see `parsers.variable`.
                  //
                  variable: function () {
                      var name, index = i;
  
                      if (input.charAt(i) === '@' && (name = $re(/^@@?[\w-]+/))) {
                          return new(tree.Variable)(name, index, env.currentFileInfo);
                      }
                  },
  
                  // A variable entity useing the protective {} e.g. @{var}
                  variableCurly: function () {
                      var curly, index = i;
  
                      if (input.charAt(i) === '@' && (curly = $re(/^@\{([\w-]+)\}/))) {
                          return new(tree.Variable)("@" + curly[1], index, env.currentFileInfo);
                      }
                  },
  
                  //
                  // A Hexadecimal color
                  //
                  //     #4F3C2F
                  //
                  // `rgb` and `hsl` colors are parsed through the `entities.call` parser.
                  //
                  color: function () {
                      var rgb;
  
                      if (input.charAt(i) === '#' && (rgb = $re(/^#([A-Fa-f0-9]{6}|[A-Fa-f0-9]{3})/))) {
                          return new(tree.Color)(rgb[1]);
                      }
                  },
  
                  //
                  // A Dimension, that is, a number and a unit
                  //
                  //     0.5em 95%
                  //
                  dimension: function () {
                      var value, c = input.charCodeAt(i);
                      //Is the first char of the dimension 0-9, '.', '+' or '-'
                      if ((c > 57 || c < 43) || c === 47 || c == 44) {
                          return;
                      }
  
                      value = $re(/^([+-]?\d*\.?\d+)(%|[a-z]+)?/);
                      if (value) {
                          return new(tree.Dimension)(value[1], value[2]);
                      }
                  },
  
                  //
                  // A unicode descriptor, as is used in unicode-range
                  //
                  // U+0??  or U+00A1-00A9
                  //
                  unicodeDescriptor: function () {
                      var ud;
  
                      ud = $re(/^U\+[0-9a-fA-F?]+(\-[0-9a-fA-F?]+)?/);
                      if (ud) {
                          return new(tree.UnicodeDescriptor)(ud[0]);
                      }
                  },
  
                  //
                  // JavaScript code to be evaluated
                  //
                  //     `window.location.href`
                  //
                  javascript: function () {
                      var str, j = i, e;
  
                      if (input.charAt(j) === '~') { j++; e = true; } // Escaped strings
                      if (input.charAt(j) !== '`') { return; }
                      if (env.javascriptEnabled !== undefined && !env.javascriptEnabled) {
                          error("You are using JavaScript, which has been disabled.");
                      }
  
                      if (e) { $char('~'); }
  
                      str = $re(/^`([^`]*)`/);
                      if (str) {
                          return new(tree.JavaScript)(str[1], i, e);
                      }
                  }
              },
  
              //
              // The variable part of a variable definition. Used in the `rule` parser
              //
              //     @fink:
              //
              variable: function () {
                  var name;
  
                  if (input.charAt(i) === '@' && (name = $re(/^(@[\w-]+)\s*:/))) { return name[1]; }
              },
  
              //
              // The variable part of a variable definition. Used in the `rule` parser
              //
              //     @fink();
              //
              rulesetCall: function () {
                  var name;
  
                  if (input.charAt(i) === '@' && (name = $re(/^(@[\w-]+)\s*\(\s*\)\s*;/))) { 
                      return new tree.RulesetCall(name[1]); 
                  }
              },
  
              //
              // extend syntax - used to extend selectors
              //
              extend: function(isRule) {
                  var elements, e, index = i, option, extendList, extend;
  
                  if (!(isRule ? $re(/^&:extend\(/) : $re(/^:extend\(/))) { return; }
  
                  do {
                      option = null;
                      elements = null;
                      while (! (option = $re(/^(all)(?=\s*(\)|,))/))) {
                          e = this.element();
                          if (!e) { break; }
                          if (elements) { elements.push(e); } else { elements = [ e ]; }
                      }
  
                      option = option && option[1];
  
                      extend = new(tree.Extend)(new(tree.Selector)(elements), option, index);
                      if (extendList) { extendList.push(extend); } else { extendList = [ extend ]; }
  
                  } while($char(","));
                  
                  expect(/^\)/);
  
                  if (isRule) {
                      expect(/^;/);
                  }
  
                  return extendList;
              },
  
              //
              // extendRule - used in a rule to extend all the parent selectors
              //
              extendRule: function() {
                  return this.extend(true);
              },
              
              //
              // Mixins
              //
              mixin: {
                  //
                  // A Mixin call, with an optional argument list
                  //
                  //     #mixins > .square(#fff);
                  //     .rounded(4px, black);
                  //     .button;
                  //
                  // The `while` loop is there because mixins can be
                  // namespaced, but we only support the child and descendant
                  // selector for now.
                  //
                  call: function () {
                      var s = input.charAt(i), important = false, index = i, elemIndex,
                          elements, elem, e, c, args;
  
                      if (s !== '.' && s !== '#') { return; }
  
                      save(); // stop us absorbing part of an invalid selector
  
                      while (true) {
                          elemIndex = i;
                          e = $re(/^[#.](?:[\w-]|\\(?:[A-Fa-f0-9]{1,6} ?|[^A-Fa-f0-9]))+/);
                          if (!e) {
                              break;
                          }
                          elem = new(tree.Element)(c, e, elemIndex, env.currentFileInfo);
                          if (elements) { elements.push(elem); } else { elements = [ elem ]; }
                          c = $char('>');
                      }
  
                      if (elements) {
                          if ($char('(')) {
                              args = this.args(true).args;
                              expectChar(')');
                          }
  
                          if (parsers.important()) {
                              important = true;
                          }
  
                          if (parsers.end()) {
                              forget();
                              return new(tree.mixin.Call)(elements, args, index, env.currentFileInfo, important);
                          }
                      }
  
                      restore();
                  },
                  args: function (isCall) {
                      var parsers = parser.parsers, entities = parsers.entities,
                          returner = { args:null, variadic: false },
                          expressions = [], argsSemiColon = [], argsComma = [],
                          isSemiColonSeperated, expressionContainsNamed, name, nameLoop, value, arg;
  
                      save();
  
                      while (true) {
                          if (isCall) {
                              arg = parsers.detachedRuleset() || parsers.expression();
                          } else {
                              parsers.comments();
                              if (input.charAt(i) === '.' && $re(/^\.{3}/)) {
                                  returner.variadic = true;
                                  if ($char(";") && !isSemiColonSeperated) {
                                      isSemiColonSeperated = true;
                                  }
                                  (isSemiColonSeperated ? argsSemiColon : argsComma)
                                      .push({ variadic: true });
                                  break;
                              }
                              arg = entities.variable() || entities.literal() || entities.keyword();
                          }
  
                          if (!arg) {
                              break;
                          }
  
                          nameLoop = null;
                          if (arg.throwAwayComments) {
                              arg.throwAwayComments();
                          }
                          value = arg;
                          var val = null;
  
                          if (isCall) {
                              // Variable
                              if (arg.value && arg.value.length == 1) {
                                  val = arg.value[0];
                              }
                          } else {
                              val = arg;
                          }
  
                          if (val && val instanceof tree.Variable) {
                              if ($char(':')) {
                                  if (expressions.length > 0) {
                                      if (isSemiColonSeperated) {
                                          error("Cannot mix ; and , as delimiter types");
                                      }
                                      expressionContainsNamed = true;
                                  }
  
                                  // we do not support setting a ruleset as a default variable - it doesn't make sense
                                  // However if we do want to add it, there is nothing blocking it, just don't error
                                  // and remove isCall dependency below
                                  value = (isCall && parsers.detachedRuleset()) || parsers.expression();
  
                                  if (!value) {
                                      if (isCall) {
                                          error("could not understand value for named argument");
                                      } else {
                                          restore();
                                          returner.args = [];
                                          return returner;
                                      }
                                  }
                                  nameLoop = (name = val.name);
                              } else if (!isCall && $re(/^\.{3}/)) {
                                  returner.variadic = true;
                                  if ($char(";") && !isSemiColonSeperated) {
                                      isSemiColonSeperated = true;
                                  }
                                  (isSemiColonSeperated ? argsSemiColon : argsComma)
                                      .push({ name: arg.name, variadic: true });
                                  break;
                              } else if (!isCall) {
                                  name = nameLoop = val.name;
                                  value = null;
                              }
                          }
  
                          if (value) {
                              expressions.push(value);
                          }
  
                          argsComma.push({ name:nameLoop, value:value });
  
                          if ($char(',')) {
                              continue;
                          }
  
                          if ($char(';') || isSemiColonSeperated) {
  
                              if (expressionContainsNamed) {
                                  error("Cannot mix ; and , as delimiter types");
                              }
  
                              isSemiColonSeperated = true;
  
                              if (expressions.length > 1) {
                                  value = new(tree.Value)(expressions);
                              }
                              argsSemiColon.push({ name:name, value:value });
  
                              name = null;
                              expressions = [];
                              expressionContainsNamed = false;
                          }
                      }
  
                      forget();
                      returner.args = isSemiColonSeperated ? argsSemiColon : argsComma;
                      return returner;
                  },
                  //
                  // A Mixin definition, with a list of parameters
                  //
                  //     .rounded (@radius: 2px, @color) {
                  //        ...
                  //     }
                  //
                  // Until we have a finer grained state-machine, we have to
                  // do a look-ahead, to make sure we don't have a mixin call.
                  // See the `rule` function for more information.
                  //
                  // We start by matching `.rounded (`, and then proceed on to
                  // the argument list, which has optional default values.
                  // We store the parameters in `params`, with a `value` key,
                  // if there is a value, such as in the case of `@radius`.
                  //
                  // Once we've got our params list, and a closing `)`, we parse
                  // the `{...}` block.
                  //
                  definition: function () {
                      var name, params = [], match, ruleset, cond, variadic = false;
                      if ((input.charAt(i) !== '.' && input.charAt(i) !== '#') ||
                          peek(/^[^{]*\}/)) {
                          return;
                      }
  
                      save();
  
                      match = $re(/^([#.](?:[\w-]|\\(?:[A-Fa-f0-9]{1,6} ?|[^A-Fa-f0-9]))+)\s*\(/);
                      if (match) {
                          name = match[1];
  
                          var argInfo = this.args(false);
                          params = argInfo.args;
                          variadic = argInfo.variadic;
  
                          // .mixincall("@{a}");
                          // looks a bit like a mixin definition.. 
                          // also
                          // .mixincall(@a: {rule: set;});
                          // so we have to be nice and restore
                          if (!$char(')')) {
                              furthest = i;
                              restore();
                              return;
                          }
                          
                          parsers.comments();
  
                          if ($re(/^when/)) { // Guard
                              cond = expect(parsers.conditions, 'expected condition');
                          }
  
                          ruleset = parsers.block();
  
                          if (ruleset) {
                              forget();
                              return new(tree.mixin.Definition)(name, params, ruleset, cond, variadic);
                          } else {
                              restore();
                          }
                      } else {
                          forget();
                      }
                  }
              },
  
              //
              // Entities are the smallest recognized token,
              // and can be found inside a rule's value.
              //
              entity: function () {
                  var entities = this.entities;
  
                  return entities.literal() || entities.variable() || entities.url() ||
                         entities.call()    || entities.keyword()  || entities.javascript() ||
                         this.comment();
              },
  
              //
              // A Rule terminator. Note that we use `peek()` to check for '}',
              // because the `block` rule will be expecting it, but we still need to make sure
              // it's there, if ';' was ommitted.
              //
              end: function () {
                  return $char(';') || peekChar('}');
              },
  
              //
              // IE's alpha function
              //
              //     alpha(opacity=88)
              //
              alpha: function () {
                  var value;
  
                  if (! $re(/^\(opacity=/i)) { return; }
                  value = $re(/^\d+/) || this.entities.variable();
                  if (value) {
                      expectChar(')');
                      return new(tree.Alpha)(value);
                  }
              },
  
              //
              // A Selector Element
              //
              //     div
              //     + h1
              //     #socks
              //     input[type="text"]
              //
              // Elements are the building blocks for Selectors,
              // they are made out of a `Combinator` (see combinator rule),
              // and an element name, such as a tag a class, or `*`.
              //
              element: function () {
                  var e, c, v, index = i;
  
                  c = this.combinator();
  
                  e = $re(/^(?:\d+\.\d+|\d+)%/) || $re(/^(?:[.#]?|:*)(?:[\w-]|[^\x00-\x9f]|\\(?:[A-Fa-f0-9]{1,6} ?|[^A-Fa-f0-9]))+/) ||
                      $char('*') || $char('&') || this.attribute() || $re(/^\([^()@]+\)/) || $re(/^[\.#](?=@)/) ||
                      this.entities.variableCurly();
  
                  if (! e) {
                      save();
                      if ($char('(')) {
                          if ((v = this.selector()) && $char(')')) {
                              e = new(tree.Paren)(v);
                              forget();
                          } else {
                              restore();
                          }
                      } else {
                          forget();
                      }
                  }
  
                  if (e) { return new(tree.Element)(c, e, index, env.currentFileInfo); }
              },
  
              //
              // Combinators combine elements together, in a Selector.
              //
              // Because our parser isn't white-space sensitive, special care
              // has to be taken, when parsing the descendant combinator, ` `,
              // as it's an empty space. We have to check the previous character
              // in the input, to see if it's a ` ` character. More info on how
              // we deal with this in *combinator.js*.
              //
              combinator: function () {
                  var c = input.charAt(i);
                  
                  if (c === '>' || c === '+' || c === '~' || c === '|' || c === '^') {
                      i++;
                      if (input.charAt(i) === '^') {
                          c = '^^';
                          i++;
                      }
                      while (isWhitespace(input, i)) { i++; }
                      return new(tree.Combinator)(c);
                  } else if (isWhitespace(input, i - 1)) {
                      return new(tree.Combinator)(" ");
                  } else {
                      return new(tree.Combinator)(null);
                  }
              },
              //
              // A CSS selector (see selector below)
              // with less extensions e.g. the ability to extend and guard
              //
              lessSelector: function () {
                  return this.selector(true);
              },
              //
              // A CSS Selector
              //
              //     .class > div + h1
              //     li a:hover
              //
              // Selectors are made out of one or more Elements, see above.
              //
              selector: function (isLess) {
                  var index = i, $re = _$re, elements, extendList, c, e, extend, when, condition;
  
                  while ((isLess && (extend = this.extend())) || (isLess && (when = $re(/^when/))) || (e = this.element())) {
                      if (when) {
                          condition = expect(this.conditions, 'expected condition');
                      } else if (condition) {
                          error("CSS guard can only be used at the end of selector");
                      } else if (extend) {
                          if (extendList) { extendList.push(extend); } else { extendList = [ extend ]; }
                      } else {
                          if (extendList) { error("Extend can only be used at the end of selector"); }
                          c = input.charAt(i);
                          if (elements) { elements.push(e); } else { elements = [ e ]; }
                          e = null;
                      }
                      if (c === '{' || c === '}' || c === ';' || c === ',' || c === ')') {
                          break;
                      }
                  }
  
                  if (elements) { return new(tree.Selector)(elements, extendList, condition, index, env.currentFileInfo); }
                  if (extendList) { error("Extend must be used to extend a selector, it cannot be used on its own"); }
              },
              attribute: function () {
                  if (! $char('[')) { return; }
  
                  var entities = this.entities,
                      key, val, op;
  
                  if (!(key = entities.variableCurly())) {
                      key = expect(/^(?:[_A-Za-z0-9-\*]*\|)?(?:[_A-Za-z0-9-]|\\.)+/);
                  }
  
                  op = $re(/^[|~*$^]?=/);
                  if (op) {
                      val = entities.quoted() || $re(/^[0-9]+%/) || $re(/^[\w-]+/) || entities.variableCurly();
                  }
  
                  expectChar(']');
  
                  return new(tree.Attribute)(key, op, val);
              },
  
              //
              // The `block` rule is used by `ruleset` and `mixin.definition`.
              // It's a wrapper around the `primary` rule, with added `{}`.
              //
              block: function () {
                  var content;
                  if ($char('{') && (content = this.primary()) && $char('}')) {
                      return content;
                  }
              },
  
              blockRuleset: function() {
                  var block = this.block();
  
                  if (block) {
                      block = new tree.Ruleset(null, block);
                  }
                  return block;
              },
              
              detachedRuleset: function() {
                  var blockRuleset = this.blockRuleset();
                  if (blockRuleset) {
                      return new tree.DetachedRuleset(blockRuleset);
                  }
              },
  
              //
              // div, .class, body > p {...}
              //
              ruleset: function () {
                  var selectors, s, rules, debugInfo;
                  
                  save();
  
                  if (env.dumpLineNumbers) {
                      debugInfo = getDebugInfo(i, input, env);
                  }
  
                  while (true) {
                      s = this.lessSelector();
                      if (!s) {
                          break;
                      }
                      if (selectors) { selectors.push(s); } else { selectors = [ s ]; }
                      this.comments();
                      if (s.condition && selectors.length > 1) {
                          error("Guards are only currently allowed on a single selector.");
                      }
                      if (! $char(',')) { break; }
                      if (s.condition) {
                          error("Guards are only currently allowed on a single selector.");
                      }
                      this.comments();
                  }
  
                  if (selectors && (rules = this.block())) {
                      forget();
                      var ruleset = new(tree.Ruleset)(selectors, rules, env.strictImports);
                      if (env.dumpLineNumbers) {
                          ruleset.debugInfo = debugInfo;
                      }
                      return ruleset;
                  } else {
                      // Backtrack
                      furthest = i;
                      restore();
                  }
              },
              rule: function (tryAnonymous) {
                  var name, value, startOfRule = i, c = input.charAt(startOfRule), important, merge, isVariable;
  
                  if (c === '.' || c === '#' || c === '&') { return; }
  
                  save();
  
                  name = this.variable() || this.ruleProperty();
                  if (name) {
                      isVariable = typeof name === "string";
                      
                      if (isVariable) {
                          value = this.detachedRuleset();
                      }
                      
                      if (!value) {
                          // prefer to try to parse first if its a variable or we are compressing
                          // but always fallback on the other one
                          value = !tryAnonymous && (env.compress || isVariable) ?
                              (this.value() || this.anonymousValue()) :
                              (this.anonymousValue() || this.value());
      
                          important = this.important();
                          
                          // a name returned by this.ruleProperty() is always an array of the form:
                          // [string-1, ..., string-n, ""] or [string-1, ..., string-n, "+"]
                          // where each item is a tree.Keyword or tree.Variable
                          merge = !isVariable && name.pop().value;
                      }
  
                      if (value && this.end()) {
                          forget();
                          return new (tree.Rule)(name, value, important, merge, startOfRule, env.currentFileInfo);
                      } else {
                          furthest = i;
                          restore();
                          if (value && !tryAnonymous) {
                              return this.rule(true);
                          }
                      }
                  } else {
                      forget();
                  }
              },
              anonymousValue: function () {
                  var match;
                  match = /^([^@+\/'"*`(;{}-]*);/.exec(current);
                  if (match) {
                      i += match[0].length - 1;
                      return new(tree.Anonymous)(match[1]);
                  }
              },
  
              //
              // An @import directive
              //
              //     @import "lib";
              //
              // Depending on our environemnt, importing is done differently:
              // In the browser, it's an XHR request, in Node, it would be a
              // file-system operation. The function used for importing is
              // stored in `import`, which we pass to the Import constructor.
              //
              "import": function () {
                  var path, features, index = i;
  
                  save();
  
                  var dir = $re(/^@import?\s+/);
  
                  var options = (dir ? this.importOptions() : null) || {};
  
                  if (dir && (path = this.entities.quoted() || this.entities.url())) {
                      features = this.mediaFeatures();
                      if ($char(';')) {
                          forget();
                          features = features && new(tree.Value)(features);
                          return new(tree.Import)(path, features, options, index, env.currentFileInfo);
                      }
                  }
  
                  restore();
              },
  
              importOptions: function() {
                  var o, options = {}, optionName, value;
  
                  // list of options, surrounded by parens
                  if (! $char('(')) { return null; }
                  do {
                      o = this.importOption();
                      if (o) {
                          optionName = o;
                          value = true;
                          switch(optionName) {
                              case "css":
                                  optionName = "less";
                                  value = false;
                              break;
                              case "once":
                                  optionName = "multiple";
                                  value = false;
                              break;
                          }
                          options[optionName] = value;
                          if (! $char(',')) { break; }
                      }
                  } while (o);
                  expectChar(')');
                  return options;
              },
  
              importOption: function() {
                  var opt = $re(/^(less|css|multiple|once|inline|reference)/);
                  if (opt) {
                      return opt[1];
                  }
              },
  
              mediaFeature: function () {
                  var entities = this.entities, nodes = [], e, p;
                  do {
                      e = entities.keyword() || entities.variable();
                      if (e) {
                          nodes.push(e);
                      } else if ($char('(')) {
                          p = this.property();
                          e = this.value();
                          if ($char(')')) {
                              if (p && e) {
                                  nodes.push(new(tree.Paren)(new(tree.Rule)(p, e, null, null, i, env.currentFileInfo, true)));
                              } else if (e) {
                                  nodes.push(new(tree.Paren)(e));
                              } else {
                                  return null;
                              }
                          } else { return null; }
                      }
                  } while (e);
  
                  if (nodes.length > 0) {
                      return new(tree.Expression)(nodes);
                  }
              },
  
              mediaFeatures: function () {
                  var entities = this.entities, features = [], e;
                  do {
                      e = this.mediaFeature();
                      if (e) {
                          features.push(e);
                          if (! $char(',')) { break; }
                      } else {
                          e = entities.variable();
                          if (e) {
                              features.push(e);
                              if (! $char(',')) { break; }
                          }
                      }
                  } while (e);
  
                  return features.length > 0 ? features : null;
              },
  
              media: function () {
                  var features, rules, media, debugInfo;
  
                  if (env.dumpLineNumbers) {
                      debugInfo = getDebugInfo(i, input, env);
                  }
  
                  if ($re(/^@media/)) {
                      features = this.mediaFeatures();
  
                      rules = this.block();
                      if (rules) {
                          media = new(tree.Media)(rules, features, i, env.currentFileInfo);
                          if (env.dumpLineNumbers) {
                              media.debugInfo = debugInfo;
                          }
                          return media;
                      }
                  }
              },
  
              //
              // A CSS Directive
              //
              //     @charset "utf-8";
              //
              directive: function () {
                  var index = i, name, value, rules, nonVendorSpecificName,
                      hasIdentifier, hasExpression, hasUnknown, hasBlock = true;
  
                  if (input.charAt(i) !== '@') { return; }
  
                  value = this['import']() || this.media();
                  if (value) {
                      return value;
                  }
  
                  save();
  
                  name = $re(/^@[a-z-]+/);
                  
                  if (!name) { return; }
  
                  nonVendorSpecificName = name;
                  if (name.charAt(1) == '-' && name.indexOf('-', 2) > 0) {
                      nonVendorSpecificName = "@" + name.slice(name.indexOf('-', 2) + 1);
                  }
  
                  switch(nonVendorSpecificName) {
                      /*
                      case "@font-face":
                      case "@viewport":
                      case "@top-left":
                      case "@top-left-corner":
                      case "@top-center":
                      case "@top-right":
                      case "@top-right-corner":
                      case "@bottom-left":
                      case "@bottom-left-corner":
                      case "@bottom-center":
                      case "@bottom-right":
                      case "@bottom-right-corner":
                      case "@left-top":
                      case "@left-middle":
                      case "@left-bottom":
                      case "@right-top":
                      case "@right-middle":
                      case "@right-bottom":
                          hasBlock = true;
                          break;
                      */
                      case "@charset":
                          hasIdentifier = true;
                          hasBlock = false;
                          break;
                      case "@namespace":
                          hasExpression = true;
                          hasBlock = false;
                          break;
                      case "@keyframes":
                          hasIdentifier = true;
                          break;
                      case "@host":
                      case "@page":
                      case "@document":
                      case "@supports":
                          hasUnknown = true;
                          break;
                  }
  
                  if (hasIdentifier) {
                      value = this.entity();
                      if (!value) {
                          error("expected " + name + " identifier");
                      }
                  } else if (hasExpression) {
                      value = this.expression();
                      if (!value) {
                          error("expected " + name + " expression");
                      }
                  } else if (hasUnknown) {
                      value = ($re(/^[^{;]+/) || '').trim();
                      if (value) {
                          value = new(tree.Anonymous)(value);
                      }
                  }
  
                  if (hasBlock) {
                      rules = this.blockRuleset();
                  }
  
                  if (rules || (!hasBlock && value && $char(';'))) {
                      forget();
                      return new(tree.Directive)(name, value, rules, index, env.currentFileInfo, 
                          env.dumpLineNumbers ? getDebugInfo(index, input, env) : null);
                  }
  
                  restore();
              },
  
              //
              // A Value is a comma-delimited list of Expressions
              //
              //     font-family: Baskerville, Georgia, serif;
              //
              // In a Rule, a Value represents everything after the `:`,
              // and before the `;`.
              //
              value: function () {
                  var e, expressions = [];
  
                  do {
                      e = this.expression();
                      if (e) {
                          expressions.push(e);
                          if (! $char(',')) { break; }
                      }
                  } while(e);
  
                  if (expressions.length > 0) {
                      return new(tree.Value)(expressions);
                  }
              },
              important: function () {
                  if (input.charAt(i) === '!') {
                      return $re(/^! *important/);
                  }
              },
              sub: function () {
                  var a, e;
  
                  if ($char('(')) {
                      a = this.addition();
                      if (a) {
                          e = new(tree.Expression)([a]);
                          expectChar(')');
                          e.parens = true;
                          return e;
                      }
                  }
              },
              multiplication: function () {
                  var m, a, op, operation, isSpaced;
                  m = this.operand();
                  if (m) {
                      isSpaced = isWhitespace(input, i - 1);
                      while (true) {
                          if (peek(/^\/[*\/]/)) {
                              break;
                          }
                          op = $char('/') || $char('*');
  
                          if (!op) { break; }
  
                          a = this.operand();
  
                          if (!a) { break; }
  
                          m.parensInOp = true;
                          a.parensInOp = true;
                          operation = new(tree.Operation)(op, [operation || m, a], isSpaced);
                          isSpaced = isWhitespace(input, i - 1);
                      }
                      return operation || m;
                  }
              },
              addition: function () {
                  var m, a, op, operation, isSpaced;
                  m = this.multiplication();
                  if (m) {
                      isSpaced = isWhitespace(input, i - 1);
                      while (true) {
                          op = $re(/^[-+]\s+/) || (!isSpaced && ($char('+') || $char('-')));
                          if (!op) {
                              break;
                          }
                          a = this.multiplication();
                          if (!a) {
                              break;
                          }
                          
                          m.parensInOp = true;
                          a.parensInOp = true;
                          operation = new(tree.Operation)(op, [operation || m, a], isSpaced);
                          isSpaced = isWhitespace(input, i - 1);
                      }
                      return operation || m;
                  }
              },
              conditions: function () {
                  var a, b, index = i, condition;
  
                  a = this.condition();
                  if (a) {
                      while (true) {
                          if (!peek(/^,\s*(not\s*)?\(/) || !$char(',')) {
                              break;
                          }
                          b = this.condition();
                          if (!b) {
                              break;
                          }
                          condition = new(tree.Condition)('or', condition || a, b, index);
                      }
                      return condition || a;
                  }
              },
              condition: function () {
                  var entities = this.entities, index = i, negate = false,
                      a, b, c, op;
  
                  if ($re(/^not/)) { negate = true; }
                  expectChar('(');
                  a = this.addition() || entities.keyword() || entities.quoted();
                  if (a) {
                      op = $re(/^(?:>=|<=|=<|[<=>])/);
                      if (op) {
                          b = this.addition() || entities.keyword() || entities.quoted();
                          if (b) {
                              c = new(tree.Condition)(op, a, b, index, negate);
                          } else {
                              error('expected expression');
                          }
                      } else {
                          c = new(tree.Condition)('=', a, new(tree.Keyword)('true'), index, negate);
                      }
                      expectChar(')');
                      return $re(/^and/) ? new(tree.Condition)('and', c, this.condition()) : c;
                  }
              },
  
              //
              // An operand is anything that can be part of an operation,
              // such as a Color, or a Variable
              //
              operand: function () {
                  var entities = this.entities,
                      p = input.charAt(i + 1), negate;
  
                  if (input.charAt(i) === '-' && (p === '@' || p === '(')) { negate = $char('-'); }
                  var o = this.sub() || entities.dimension() ||
                          entities.color() || entities.variable() ||
                          entities.call();
  
                  if (negate) {
                      o.parensInOp = true;
                      o = new(tree.Negative)(o);
                  }
  
                  return o;
              },
  
              //
              // Expressions either represent mathematical operations,
              // or white-space delimited Entities.
              //
              //     1px solid black
              //     @var * 2
              //
              expression: function () {
                  var entities = [], e, delim;
  
                  do {
                      e = this.addition() || this.entity();
                      if (e) {
                          entities.push(e);
                          // operations do not allow keyword "/" dimension (e.g. small/20px) so we support that here
                          if (!peek(/^\/[\/*]/)) {
                              delim = $char('/');
                              if (delim) {
                                  entities.push(new(tree.Anonymous)(delim));
                              }
                          }
                      }
                  } while (e);
                  if (entities.length > 0) {
                      return new(tree.Expression)(entities);
                  }
              },
              property: function () {
                  var name = $re(/^(\*?-?[_a-zA-Z0-9-]+)\s*:/);
                  if (name) {
                      return name[1];
                  }
              },
              ruleProperty: function () {
                  var c = current, name = [], index = [], length = 0, s, k;
                  
                  function match(re) {
                      var a = re.exec(c);
                      if (a) {
                          index.push(i + length);
                          length += a[0].length;
                          c = c.slice(a[1].length);
                          return name.push(a[1]);
                      }
                  }
  
                  match(/^(\*?)/);
                  while (match(/^((?:[\w-]+)|(?:@\{[\w-]+\}))/)); // !
                  if ((name.length > 1) && match(/^\s*((?:\+_|\+)?)\s*:/)) {
                      // at last, we have the complete match now. move forward, 
                      // convert name particles to tree objects and return:
                      skipWhitespace(length);
                      if (name[0] === '') {
                          name.shift();
                          index.shift();
                      }
                      for (k = 0; k < name.length; k++) {
                          s = name[k];
                          name[k] = (s.charAt(0) !== '@')
                              ? new(tree.Keyword)(s)
                              : new(tree.Variable)('@' + s.slice(2, -1), 
                                  index[k], env.currentFileInfo);
                      }
                      return name;
                  }
              }
          }
      };
      return parser;
  };
  less.Parser.serializeVars = function(vars) {
      var s = '';
  
      for (var name in vars) {
          if (Object.hasOwnProperty.call(vars, name)) {
              var value = vars[name];
              s += ((name[0] === '@') ? '' : '@') + name +': '+ value +
                      ((('' + value).slice(-1) === ';') ? '' : ';');
          }
      }
  
      return s;
  };
  