repopulating the fleet one key feature of alien invasion is that the aliens are relentlessevery time the fleet is destroyeda new fleet should appear to make new fleet of aliens appear after fleet has been destroyedfirst check to see whether the group aliens is empty if it iswe call create_fleet(we'll perform this check in update_bullets(because that' where individual aliens are destroyedgame_ functions py def update_bullets(ai_settingsscreenshipaliensbullets)--snip-check for any bullets that have hit aliens if soget rid of the bullet and the alien collisions pygame sprite groupcollide(bulletsalienstruetrueu if len(aliens= destroy existing bullets and create new fleet bullets empty(create_fleet(ai_settingsscreenshipaliensat we check whether the group aliens is empty if it iswe get rid of any existing bullets by using the empty(methodwhich removes all the remaining sprites from group we also call create_fleet()which fills the screen with aliens again