docs: 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));
              }