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