I now have a working Game Boy character controller where the D-pad causes a little box to move freely around the screen. That's not really something I needed, but it's now something I have.
https://git.gay/mcc/mermaid-nil/src/branch/project-twice/project/twice/twice.l0
As you perhaps can see above, I'm developing this in my own macro language, which theoretically means when I move to the next bit I can introduce high-level abstractions, but only if I change the language to add macros. I need to learn what a "quasiquote" is I guess.