What would a shoot-em-up be without something to shoot at? We're going to add something to fix that: enemies!
The engine for enemies is based on pretty much the same idea as the bullet engine. You've got an array full of them, and they all move in some manner. We should probably keep the two arrays separate, since bullets and enemies can work in very different ways. L5 is a nice place to store the new array, assuming you won't be dealing with 32 or more enemies at a time (which would overflow the 128 bytes you get in L5).
