when you're beginning new phase of development on larger projectit' always good idea to revisit your plan and clarify what you want to accomplish with the code you're about to write in this we willexamine our code and determine if we need to refactor before implementing new features add single alien to the top-left corner of the screen with appropriate spacing around it use the spacing around the first alien and the overall screen size to determine how many aliens can fit on the screen we'll write loop to create aliens to fill the upper portion of the screen make the fleet move sideways and down until the entire fleet is shot downan alien hits the shipor an alien reaches the ground if the whole fleet is shot downwe'll create new fleet if an alien hits the ship or the groundwe'll destroy the ship and create new fleet limit the number of ships the player can useand end the game when the player has used up the allotment of ships we'll refine this plan as we implement featuresbut this is sufficient to start with you should also review code when you're about to begin working on new series of features in project because each new phase typically makes project more complexit' best to clean up cluttered or inefficient code although we don' have much cleanup to do right now because we've been refactoring as we goit' annoying to use the mouse to close the game each time we run it to test new feature let' quickly add keyboard shortcut to end the game when the user presses qgame_ functions py def check_keydown_events(eventai_settingsscreenshipbullets)--snip-elif event key =pygame k_qsys exit(in check_keydown_events(we add new block that ends the game when is pressed this is fairly safe change because the key is far from the arrow keys and the spacebarso it' unlikely player will accidentally press and quit the game nowwhen testingyou can press to close the game rather than using your mouse to close the window creating the first alien placing one alien on the screen is like placing ship on the screen the behavior of each alien is controlled by class called alienwhich we'll structure like the ship class we'll continue using bitmap images for simplicity you can find your own image for an alien or use the one