Prolog mode

[Andrea Crotti]

I followed exactly the instructions from
http://bruda.ca/emacs-prolog/install.html
and copied and loaded the file, but when I try to use the prolog-mode I
get a nice
--8<---------------cut here---------------start------------->8---
if: Symbol's value as variable is void: prolog-mode-map-swi
--8<---------------cut here---------------end--------------->8---

but 'swi is correct, what's wrong with this configuration?

--8<---------------cut here---------------start------------->8---
  (autoload 'run-prolog "prolog" "Start a Prolog sub-process." t)
  (autoload 'prolog-mode "prolog" "Major mode for editing Prolog programs." t)
  (autoload 'mercury-mode "prolog" "Major mode for editing Mercury programs." t)
  (setq prolog-system 'swi)
  (add-to-list 'auto-mode-alist '("\\.pl$" . prolog-mode))
--8<---------------cut here---------------end--------------->8---

Thanks,
Andrea