def modify_write(self, write_pb, **unused_kwargs):
        """Modify a ``Write`` protobuf based on the state of this write option.

        The ``last_update_time`` is added to ``write_pb`` as an "update time"
        precondition. When set, the target document must exist and have been
        last updated at that time.

        Args:
            write_pb (google.cloud.firestore_v1beta1.types.Write): A
                ``Write`` protobuf instance to be modified with a precondition
                determined by the state of this option.
            unused_kwargs (Dict[str, Any]): Keyword arguments accepted by
                other subclasses that are unused here.
        """
        current_doc = types.Precondition(update_time=self._last_update_time)
        write_pb.current_document.CopyFrom(current_doc)