Prolog uses .pl and because of the Perl conflict occasionally .pro: http://www.swi-prolog.org/pldoc/man?section=fileext

I still found a

    autocmd Filetype pl set syntax=prolog

in my vim config from my Prolog days.

Exactly. Perl has insensitively hijacked it.

Now you know how forth programmers feel about F#. (.fs)