code: def _rollback(self):
          
          if not self.in_progress:
              raise ValueError(_CANT_ROLLBACK)
  
          try:
              # NOTE: The response is just ``google.protobuf.Empty``.
              self._client._firestore_api.rollback(
                  self._client._database_string,
                  self._id,
                  metadata=self._client._rpc_metadata,
              )
          finally:
              self._clean_up()
docs: Roll back the transaction.
  
          Raises:
              ValueError: If no transaction is in progress.