code: def fail(self, uid):
          
          self._redis.pipeline()\
                     .lrem(self.ACKBUF_KEY, 0, uid)\
                     .lrem(self.BACKUP, 0, uid)\
                     .lpush(self.QUEUE_KEY, uid)\
                     .execute()
docs: Report item as not successfully consumed.
  
          Removes uid from ackbuffer and re-enqueues it.