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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _add_complete_key_entity_pb(self):
 """Adds a new mutation for an entity with a completed key. :rtype: :class:`.entity_pb2.Entity` :returns: The newly created entity protobuf that will be updated and sent with a commit. """

        # We use ``upsert`` for entities with completed keys, rather than
        # ``insert`` or ``update``, in order not to create race conditions
        # based on prior existence / removal of the entity.
        new_mutation = _datastore_pb2.Mutation()
        self._mutations.append(new_mutation)
        return new_mutation.upsert