In the example [ping.py](https://github.com/moos-tutorials/python-moos-tutorials/blob/master/01-active-queues/ping.py), if you take away the line : ```python self.set_on_mail_callback(self.__on_new_mail) ``` the `on_pong_message` active queue is never activated. I doubt that it's the `self.fetch()` that actually makes it work.
