code: def _add_write_pbs(self, write_pbs):
          
          if self._read_only:
              raise ValueError(_WRITE_READ_ONLY)
  
          super(Transaction, self)._add_write_pbs(write_pbs)
docs: Add `Write`` protobufs to this transaction.
  
          Args:
              write_pbs (List[google.cloud.proto.firestore.v1beta1.\
                  write_pb2.Write]): A list of write protobufs to be added.
  
          Raises:
              ValueError: If this transaction is read-only.