code: def modify_write(self, write_pb, **unused_kwargs):
          
          current_doc = types.Precondition(exists=self._exists)
          write_pb.current_document.CopyFrom(current_doc)
docs: 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.