code: def start(self):
          
          # 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
docs: Starts all the actors to run a system of process applications.