bug#67008: 30.0.50; Multiple major mode parents

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

>> Having read a bit further, I'm now not sure that `merge-ordered-lists`
>> implements C3.  I suspect it provides a useful primitive with which to
>> implement C3 but whether it gives the same result as C3 or not depends
>> on how we call it.
> Yes, the output seems very sensitive about the input order:
>
>  (B A) (C A) (D B) (E D C) -> (E D B C A)
>
>  (E D C) (B A) (C A) (D B) -> (E D C B A)

Yes, that's on purpose: when the dependencies expressed by the
(sub)lists don't enforce a unique solution, the order between the
(sub)lists is used to choose between the available options.

> Maybe that's obvious for someone who knows how to use it.

I improved the docstring to try and clarify.

> It could probably be used as a general-purpose topo-sorter but it's not
> a very efficient one (at least quadratic, possibly cubic).

Also it doesn't start from a graph but from a lists of lists (because
for uses in method inheritance it's very desirable that the ordering be
"monotonic": if A comes before B in the linearization of class C's
parents then the same should be true in the linearization of all its
subclasses).


        Stefan