code: def _commit(self):
          
          if not self.in_progress:
              raise ValueError(_CANT_COMMIT)
  
          commit_response = _commit_with_retry(self._client, self._write_pbs, self._id)
  
          self._clean_up()
          return list(commit_response.write_results)
docs: Transactionally commit the changes accumulated.
  
          Returns:
              List[google.cloud.proto.firestore.v1beta1.\
                  write_pb2.WriteResult, ...]: The write results corresponding
              to the changes committed, returned in the same order as the
              changes were applied to this transaction. A write result contains
              an ``update_time`` field.
  
          Raises:
              ValueError: If no transaction is in progress.