Again, I’ve figured this out, but thought it worth recording, as the compiler might, in an ideal world, have issued a warning to prevent me making the error.
I decided to implement an UNSCREW action.
I set out
o an UNSCREW action
o an UNSCREW a noun action
o an UNSCREW noun with a second noun action
It compiled without comment, yet when I played it, the Unscrew action worked, the Unscrew noun with a second noun worked, but my Unscrew noun action was ignored, and the game reported that nothing had happened.
I eventually realised, by inspecting the Index in a different game, that, in the built-in rules, Unscrew was a synonym of Turn.
The solution was, of course, to declare 'Understand the command ‘Unscrew’ as something new".