code: def on_enter_stage(self):
          
          with self.world._unlock_temporarily():
              self.forum.connect_everyone(self.world, self.actors)
  
          # 1. Setup the forum.
  
          self.forum.on_start_game()
  
          # 2. Setup the world.
  
          with self.world._unlock_temporarily():
              self.world.on_start_game()
  
          # 3. Setup the actors.  Because this is done after the forum and the  
          #    world have been setup, this signals to the actors that they can 
          #    send messages and query the game world as usual.
  
          num_players = len(self.actors) - 1
  
          for actor in self.actors:
              actor.on_setup_gui(self.gui)
  
          for actor in self.actors:
              actor.on_start_game(num_players)
docs: Prepare the actors, the world, and the messaging system to begin 
          playing the game.
          
          This method is guaranteed to be called exactly once upon entering the 
          game stage.