You have to keep testing it and go back and forth to the code

nav_game.py

@@ -1,4 +1,9 @@
-# nav_game.py
-# ------------
-# By MWC Contributors
-# This class implements a simple game where a spaceship avoids asteroids.
+from retro.game import Game
+from spaceship import Spaceship
+from asteroid_spawner import AsteroidSpawner
+
+board_size = (25, 25)
+ship = Spaceship(board_size)
+spawner = AsteroidSpawner(board_size)
+game = Game([ship, spawner], {"score": 0}, board_size=board_size)
+game.play()