code: function(){
                  /*
                   * expression
                   *  : '(' S* media_feature S* [ ':' S* expr ]? ')' S*
                   *  ;
                   */
                  var tokenStream = this._tokenStream,
                      feature     = null,
                      token,
                      expression  = null;
  
                  tokenStream.mustMatch(Tokens.LPAREN);
                  feature = this._media_feature();
                  this._readWhitespace();
  
                  if (tokenStream.match(Tokens.COLON)){
                      this._readWhitespace();
                      token = tokenStream.LT(1);
                      expression = this._expression();
                  }
  
                  tokenStream.mustMatch(Tokens.RPAREN);
                  this._readWhitespace();
                  return new MediaFeature(feature, (expression ? new SyntaxUnit(expression, token.startLine, token.startCol) : null));
              }
docs: Note: in CSS3 Media Queries, this is called "expression".
  Renamed here to avoid conflict with CSS3 Selectors
  definition of "expression". Also note that "expr" in the
  grammar now maps to "expression" from CSS3 selectors.
  @method _media_expression
  @private