docs: def add_data(self, id, key, value):
          """Add new data item.
  
          :param str id: Entry id within ``SDfile``.
          :param str key: Data item key.
          :param str value: Data item value.
          :return: None.
          :rtype: :py:obj:`None`.
          """
          self[str(id)]['data'].setdefault(key, [])
          self[str(id)]['data'][key].append(value)