Consider the definition of a stack - last-in, first-out. If you load a bunch of stuff into a stack, and then take it all out again, you now have your original items *in reverse order*.

Take the points (1,1), (1,-1), (-1,1) and (-1,-1). Stretch the set to however wide/tall you need, then run all four through

x *= cos(a);
y *= sin(a);

It's the same principle as in the...