anchor.diff:              before = new Set.from(results[0].map((d) => d['_id']));
               after = new Set.from(results[1].map((d) => d['_id']));
               diff = [];
  -  
  +
               beforeOrAfter.forEach((record) {
                 assert(record['version']>version);
  -              
  +
                 _stripCleanVersion(record['before']);
                 _stripCleanVersion(record['after']);
  -              
  +
                 if(before.contains(record['_id']) && after.contains(record['_id']))
                 {
                   // record was changed
  
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};
  