anchor.diff:     */
     Future<Map> data() {
       Map selector = _selectorList.isEmpty ? {} : {AND: _selectorList};
  -    print(selector);
       return collection.find(selector).toList().then((data) {
         var version = data.length == 0 ? 0 :
           data.map((item) => item['__clean_version']).reduce(max);
  
positive.code:           "version" : nextVersion
          }),
        onError: (e) {
          print(e);
          // Errors thrown by MongoDatabase are Map objects with fields err, code,
          // ...
          return _release_locks().then((_) {
  
negative.code:         }
        },
        onError: (e) {
          // Errors thrown by MongoDatabase are Map objects with fields err, code,
          // ...
          return _release_locks().then((_) {
  