Cool project! But one nitpick.
K&R C has no concept of THEN. That's a peculiarity of the ZIP source code, defined as a pure no-op:
#define THEN
Oh wow, thank you for the clarification. I completely didn't even consider to audit for that in the ZIP source (though the ALL CAPS maybe should have been my hint). I'll update the post with this and another small thing another player found.