abstract class DataProvider {
 /** Returns collection of items in the following form:
  * {'data': [List<Map>] data, 'version': [num] version_num}
  */
 Future<Map> data();
 /** Returns collection of items in the following form:
  *  {'diff': [List<Map>]} or
  *  {'diff': null, 'data': [List<Map>] data, 'version': [num] version_num}
  *
  *  If 'diff' value is not null, items in the list are of following form:
  *  {'action': 'add'/'change',
  *   '_id': 'value0',
  *   'author': 'Some String',
  *   'data': {'_id': 'value0', 'field1': 'value1', 'field2': 'value2', ...}
  *   'version': 5}
  *
  *  or
  *
  *  {'action': 'remove',
  *   '_id': 'value0',
  *   'author': 'Some String',
  *   'version': 5}
  *
  *  In case of 'add', value of 'data' is a [Map] representing new data that
  *  was added. In case of 'change', value of 'data' is a [Map] containing new
  *  key-value pairs and/or pairs of already existing keys and updated values.
  */
 Future<Map> diffFromVersion(num version);
 Future add(Map data, String author);
 Future change(String_id, Map change, String author);
 Future remove(String id, String author);
}