anchor.diff:    return Error.safeToString(input);
   }
   
  -void _log(Object o) {
  +void _log(Object o, [List<String> subContexts]) {
     String safe;
   
     try {
  
positive.code:       if(option != null && option.allowed != null && !option.allowed.isEmpty) {
          assert(!option.isFlag);
  
          _log('completing all allowed value for option "${option.name}"');
  
          return option.allowed.toList();
        }
  
  
      } else {
        _log('completing the name of options starting with "$removedItem"');
  
        return parserOptionCompletions
            .where((String option) => option.startsWith(removedItem))
  
negative.code:   final validSubSet = subsetTuple.item1;
    final subsetResult = subsetTuple.item2;
  
    sublog('valid subset: ${_helpfulToString(validSubSet)}');
  
    /*
     * CASE: we have a command
  