bug#70589: [PATCH] Refine the Custom type of generated '*-modes' options

[Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

>> > This makes it easier for users to customize generated '*-modes' options,
>> > such as 'global-completion-preview-modes', via the Custom interface.
>> 
>> This kind of discussion is not useful with Stefan (CC'ed) on board.
>                                         ^^^^
> Oops, I meant "without", of course.  Sorry.

🙂

FWIW, the latest patch looks fine to me, but really this is Lars's code,
so you don't need me "on board", really.

Also, while I'm here: I think part of the "problem" is in the semantics:
anything after a t or nil is ignored, which is hard to explain via
a "single small line" of text.

We could imagine extending the semantics so that you can use `((not a)
. t)` instead, in which case this meaning of `t` can be shared with the
"main one" (instead of having two `t` cases and two `nil` cases) in the
sense that the syntax would be:

    PRED can be:
    t                        Always accept
    nil                      Always decline
    ((not . MODES) . PRED)   Decline if in MODES, otherwise like PRED
    (MODE . PRED)            Accept if in MODE, otherwise like PRED

but then we couldn't use the `repeat` Custom type and instead we'd have
to implement an ad-hoc type for it, I believe.


        Stefan