anchor.diff:  
       Set before, after;
       List beforeOrAfter, diff;
  -    
  +
        return _maxVersion.then((maxVersion) {
           return _collectionHistory.find(beforeOrAfterSelector).toList()
           .then((result) {
  
positive.code:     _lock.remove({'_id': collection.collectionName})).then((_) =>
      true);
    }
    
    void _stripCleanVersion(dynamic data) {
      if (data is Iterable) {
        data.forEach((Map item) {
  
negative.code:       //return _maxVersion.then((version) => {'data': data, 'version': version});
        var version = data.length == 0 ? 0 :
          data.map((item) => item['__clean_version']).reduce(max);
  
        _stripCleanVersion(data);
  
        return {'data': data, 'version': version};
  