Damn, I completely missed the existence SRFI 14 and Arthur didn't tip me
off to it either :) Sorry about that! Indeed, the ASCII sets can be
derived exactly as you write. According to the Gauche manual, SRFI 14 is
being incorporated into R7RS-large as well. So it'd be doubly good to
interoperate with it.
Sorry, I didn't think of SRFI 14, either. We have enough SRFIs now that I don't always remember them all. And your SRFI seemed so simple conceptually that it didn't even occur to me that it would be related to another — just to the corresponding Unicode procedures.