text: var Type = require("@kaoscript/runtime").Type;
  module.exports = function(expect) {
  	let foo = (() => {
  		return function(x = null) {
  			if(x !== null && !Type.isNumber(x)) {
  				throw new TypeError("'x' is not of type 'Number?'");
  			}
  			return [x];
  		};
  	})();
  	expect(foo()).to.eql([null]);
  	expect(foo(1)).to.eql([1]);
  	expect(() => {
  		return foo("foo");
  	}).to.throw();
  	expect(foo(1, 2)).to.eql([1]);
  	expect(() => {
  		return foo("foo", 1);
  	}).to.throw();
  };