Ah, I see. Yes, I mentioned in the post that I was dropping some magic numbers. Mu is intended to be read in a text editor, and I didn't want to scare people too much while reading the post in a browser. Slightly evil, I know.

The concrete rules are illustrated at https://github.com/akkartik/mu/blob/469a1a9ace9b290658beb9d0.... In this case `%<reg>` always translates to `3/mod <reg-code>/rm32`.