    def on_rollback(self, bigchain, new_height):
        """Override to clean up the database artifacts possibly created
           in `on_approval`. Part of the `end_block`/`commit` crash recovery.
        """
        raise NotImplementedError
Override to clean up the database artifacts possibly created
in `on_approval`. Part of the `end_block`/`commit` crash recovery.
bigchaindb/elections/election.py