bug#45747: 27.1; mercury-mode font lock bug

[Lars Ingebrigtsen <larsi@gnus.org>]

k3tu0isui@gmail.com writes:

> Maybe prolog-mode-variables has already been called, but maybe due to
> (setq-local prolog-system 'mercury) not being set, some variables like
> prolog-determinism-specifiers-i and prolog-types-i are not being set
> to mercury values. So quite a bit of keywords are ignored when
> syntax-highlighting happens.

Ah, I missed that `prolog-find-value-by-system' (called by that
function) depended on `prolog-system', so I think your patch is correct,
and I've now applied it to Emacs 28.  (With the `derived-mode-p' tweak.)

This change was small enough to apply without assigning copyright to the
FSF, but for future patches you want to submit, it might make sense to
get the paperwork started now, so that subsequent patches can be applied
speedily. Would you be willing to sign such paperwork?

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no