The problem is that e.g. a window needs to both add *and* subtract something
from the object it is to be placed into. There is no CSG operation that can
achieve both at the same time.

What I'd do is make two macros per element: One that provides a shape to be cut
away using difference, and another one that provides a shape to be added using
union.

This is not perfectly OO-style of course, but then again, oo design patterns is
not something POV SDL ever claimed to be good at.

However, you might use a 2-element array to hold the two shapes. Maybe this gets
you closer to your preferred coding style.

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