<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def start(self):
 """Start the listener. This starts up a background thread to monitor the queue for items to process. """

        self._thread = t = threading.Thread(target=self._monitor)
        t.setDaemon(True)
        t.start()