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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):
 """ Starts all the actors to run a system of process applications. """

        # Subscribe to broadcast prompts published by a process
        # application in the parent operating system process.
        subscribe(handler=self.forward_prompt, predicate=self.is_prompt)

        # Initialise the system actor.
        msg = SystemInitRequest(
            self.system.process_classes,
            self.infrastructure_class,
            self.system.followings,
            self.pipeline_ids
        )
        response = self.actor_system.ask(self.system_actor, msg)

        # Keep the pipeline actor addresses, to send prompts directly.
        assert isinstance(response, SystemInitResponse), type(response)

        assert list(response.pipeline_actors.keys()) == self.pipeline_ids, (
            "Configured pipeline IDs mismatch initialised system {} {}").format(
            list(self.pipeline_actors.keys()), self.pipeline_ids
        )

        self.pipeline_actors = response.pipeline_actors