<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _on_cancelok(self, cancel_frame):
 """ Called when the server acknowledges a cancel request. Args: cancel_frame (pika.spec.Basic.CancelOk):
 The cancelok frame from the server. """

        _log.info("Consumer canceled; returning all unprocessed messages to the queue")
        self._channel.basic_nack(delivery_tag=0, multiple=True, requeue=True)