human
Solve the following python programming problem given the following code:
def on_start(channel, event) : 
    'Callback for StasisStart events.

    On new channels, answer, play demo-congrats, and register a DTMF listener.

    :param channel: Channel DTMF was received from.
    :param event: Event.
    ' 
    channel.answer () 
    playback = channel.play (media = 'sound:demo-congrats') 
    def on_dtmf(channel, event) : 
        'Callback for DTMF events.

        DTMF events control the playback operation.

        :param channel: Channel DTMF was received on.
        :param event: Event.
        ' 
        digit = event ['digit'] 
        if (digit == '5') : 
            playback.control (operation = 'pause') 
else : 
            if (digit == '8') : 
                playback.control (operation = 'unpause') 
else : 
                if (digit == '4') : 
                    playback.control (operation = 'reverse') 
else : 
                    if (digit == '6') : 
                        playback.control (operation = 'forward') 
else : 
                        if (digit == '2') : 
                            playback.control (operation = 'restart') 
else : 
                            if (digit == '#') : 
                                playback.stop () 
                                channel.continueInDialplan () 
else : 
                                ((print >> sys.stderr), ('Unknown DTMF %s' % digit)) 
channel.on_event ('ChannelDtmfReceived', event) 

    
    

    Fix the buggy line: channel.on_event ('ChannelDtmfReceived', event)
assistant
#Here is Python code to fix the buggy line: channel.

channel.on_event ('ChannelDtmfReceived', def on_dtmf()