Thanks, that looks promising, I will take a closer look.

Thanks for your response. One of the constraints is that it needs to be an entirely Common Lisp solution; external tools or C libraries via FFI can’t be part of the solution.

You’re right that macros or other C constructs can complicate the picture. Whether or not that’s an issue depends on the assumptions that tools writing XPM files make. At this point I’m willing to just get past the comments issue and then see how far that gets me.