code: def rollback(cls, bigchain, new_height, txn_ids):
          
  
          # delete election records for elections initiated at this height and
          # elections concluded at this height
          bigchain.delete_elections(new_height)
  
          txns = [bigchain.get_transaction(tx_id) for tx_id in txn_ids]
  
          elections = cls._get_votes(txns)
          for election_id in elections:
              election = bigchain.get_transaction(election_id)
              election.on_rollback(bigchain, new_height)
docs: Looks for election and vote transactions inside the block and
             cleans up the database artifacts possibly created in `process_blocks`.
  
             Part of the `end_block`/`commit` crash recovery.