code: def main(car_factory, extra_engine):
      
      car1 = car_factory(serial_number=1)
      car2 = car_factory(serial_number=2, engine=extra_engine)
  
      assert car1.serial_number == 1 and car2.serial_number == 2
      assert car1.engine is not car2.engine
      assert car2.engine is extra_engine
docs: Run application.