Modify a ``Write`` protobuf based on the state of this write option.

        If:

        * ``exists=True``, adds a precondition that requires existence
        * ``exists=False``, adds a precondition that requires non-existence

        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.

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

        If:

        * ``exists=True``, adds a precondition that requires existence
        * ``exists=False``, adds a precondition that requires non-existence

        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(exists=self._exists)
        write_pb.current_document.CopyFrom(current_doc)