docs: def get_descriptions(self, description_type):
          """
          Gets the descriptions for specified type.
          When complete the callback is called with a list of descriptions
          """
          (desc_type, max_units) = description_type
          results = [None] * max_units
          self.elk._descriptions_in_progress[desc_type] = (max_units,
                                                           results,
                                                           self._got_desc)
          self.elk.send(sd_encode(desc_type=desc_type, unit=0))