anchor.diff:        return inner.separatedBy(_buildChoice(_right)).map((sequence) {
           var result = sequence.last;
           for (var i = sequence.length - 2; i > 0; i -= 2) {
  -          result =
  -              sequence[i].action(sequence[i - 1], sequence[i].operator, result);
  +          final expressionResult = sequence[i] as ExpressionResult;
  +          result = expressionResult.action(
  +              sequence[i - 1], expressionResult.operator, result);
           }
           return result;
         });
  
positive.code: 
    /// Adds a postfix operator [parser]. Evaluates the optional [action] with the
    /// parsed `value` and `operator`.
    void postfix(Parser parser, [Function action]) {
      action ??= (value, operator) => [value, operator];
      _postfix.add(parser.map((operator) => ExpressionResult(operator, action)));
    }
  
negative.code: library petitparser.core.expression.group;
  
  import 'package:petitparser/src/core/combinators/choice.dart';
  import 'package:petitparser/src/core/combinators/sequence.dart';
  import 'package:petitparser/src/core/expression/result.dart';
  