From Assignment #1 Handout:
A simple video game we’ll call Space Fights. In this game, the human player controls the location of a spaceship. We represent the spaceship only by its main entry door and thus, the location of the spaceship indicates the location of its door. In addition to changing the door location, the player can also expand or contract the size of this door. The player accumulates points by rescuing as many astronauts as possible (who fight with the aliens) by opening the door and letting them in to the spaceship.