[Redneck]

I checked... only one copy of user_routines_fast.c
Also, after checking every file in the workspace, I only found InterruptVectorLow declared in ONE place, where it's supposed to be in user_routines_fast.c

I re-downloaded the default code, copy/pasted all the changes we'd made into the new copy of the default EXACTLY as they were in our previous code, and it magically compiles now.