anchor.diff:  
     Parser or(Parser other) {
       var parsers = new List.from(_parsers);
  -    parsers.addLast(other);
  +    parsers.add(other);
       return new _ChoiceParser(parsers);
     }
   
  
positive.code: 
    Result _parse(Context context) {
      var current = context;
      var elements = new List(_parsers.length);
      for (var i = 0; i < _parsers.length; i++) {
        var result = _parsers[i]._parse(current);
        if (result.isFailure) {
  
negative.code: 
    Parser seq(Parser other) {
      var parsers = new List.from(_parsers);
      parsers.add(other);
      return new _SequenceParser(parsers);
    }
  
  