anchor.diff:     * parsed `value` and `operator`.
      */
     void postfix(Parser operator, [void action(value, operator)]) {
  -    _postfix.add(new _PostfixOperator(operator, action));
  +    _operators.putIfAbsent(_POSTFIX, () => new List())
  +        .add(new _ExpressionOperator(operator, action, (operators, next) {
  +          return new _SequenceParser([next, operators]).map((results) {
  +            return results.last.action(results.first, results.last.value);
  +          });
  +        }));
     }
   
     /**
  
positive.code:    * or to recurse into another group.
     */
    void primitive(Parser parser, [void action(value)]) {
      _primitive.add(new _PrimitiveOperator(parser, action));
    }
  
    /**
  
negative.code: }
  
  class _ExpressionOperator {
    final Parser parser;
    final Function action;
    final Function combinator;
    _ExpressionOperator(this.parser, this.action, this.combinator);
  }
  
  class _ExpressionOperatorResult {
    final dynamic value;
    final Function action;
    _ExpressionOperatorResult(this.value, this.action);
  }