<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def nack(self, items):
 """Explicitly deny receipt of messages. Args: items(Sequence[NackRequest]):
 The items to deny. """

        self.modify_ack_deadline(
            [requests.ModAckRequest(ack_id=item.ack_id, seconds=0) for item in items]
        )
        self.drop([requests.DropRequest(*item) for item in items])