bug#46917: 28.0.50; error when unloading feature after loading elp

[Stefan Monnier <monnier@iro.umontreal.ca>]

>> It's because it's supposed to be a "list of qualifiers".
> The three ones that are supported, though, are mutually exclusive, I think?
>> Strictly speaking, the set of possible qualifiers can be changed by
>> adding a new method to `cl-generic-combine-methods`.
> Perhaps that should be mentioned somewhere.  :-)

Except that it's a bad hook, really, because there's not much on
which you can "dispatch" nor can you satisfactorily use the result of
`cl-call-next-method`, so all you can do is override the default with
a fresh new implementation.

I think it's just barely sufficient so you can use a different method
combination for specific generic functions (i.e. we could implement
something like CLOS's support for specifying in `defgeneric`
a particular method combination to use for that generic function), but
it's not sufficient to be able to extend the set of qualifiers supported
"by default".


        Stefan