> By the way, the syntax and function of my initially proposed
> \keys_subgroups_set:nnn could be roughly captured by the following
> code (though I doubt it would add anything useful any more)

What I meant to say here is not that the new proposal does roughly
what \keys_subgroups_set:nnn actually did, but what I had wanted it to
do.

Come to think of it, though, it would perhaps be useful to have some
sort of ability to search across subgroups (independently of the
key-filtering application). Perhaps something corresponding to the
pgfkeys ’.search also’ handler.