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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:

def send_stop_audio(self):
        '''
        Stop an audio streaming session
        '''
        assert self._session_id != VoiceService.SESSION_ID_INVALID

        self._pebble.send_packet(AudioStream(session_id=self._session_id, data=StopTransfer()))