anchor.diff:    Result parseOn(Context context) {
       var buffer = context.buffer;
       var position = context.position;
  -    if (position < buffer.length && _matcher(buffer.codeUnitAt(position))) {
  +    if (position < buffer.length && _matcher.match(buffer.codeUnitAt(position))) {
         return context.success(buffer[position], position + 1);
       }
       return context.failure(_message);
  
positive.code:   const _UppercaseCharMatcher();
  
    @override
    bool call(int value) => 65 <= value && value <= 90;
  
  }
  
  
negative.code:   const _DigitCharMatcher();
  
    @override
    bool match(int value) => 48 <= value && value <= 57;
  
  }
  
  