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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 start(self, exit=True):
 """Invoked the application callable method and start the ``arbiter`` if it wasn't already started. It returns a :class:`~asyncio.Future` called back once the application/applications are running. It returns ``None`` if called more than once. """

        on_start = self()
        actor = arbiter()
        if actor and on_start:
            actor.start(exit=exit)
            if actor.exit_code is not None:
                return actor.exit_code
        return on_start