Gauche's Shiro Kawai is working through it:
https://nitter.1d4.us/anohana/status/1372375694999658497
My Japanese is nicht so gut, but roughly,
Software Design for Flexibility, 2.3: Up until now compose was only defined with two arguments but without saying anything they passed it three.
Ah, so the idea is to write it in Exercise 2.5c?
Old /prog/ celebrity The Ginger (who still refuses to release Croma) also tweeted about it: