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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 on_unsubscribe(self):
 # type: () -> Callable """Decorate a callback funtion to handle unsubscribtions. **Usage:**:: @mqtt.unsubscribe() def handle_unsubscribe(client, userdata, mid) print('Unsubscribed from topic (id: {})' .format(mid)') """

        def decorator(handler):
            # type: (Callable) -> Callable
            self.client.on_unsubscribe = handler
            return handler

        return decorator