FAT is 8 + 3, so it still had space for .pro as well.

Prolog is older than Perl. Perl usurped the .pl extension. Some are trying to claim it back.

Yes, it looks like modern Prolog systems using ".pl". I checked Ciao, GNU Prolog (which recognizes ".pl" and ".pro"), SICStus Prolog (also ".pl" and .pro"), SWI-Prolog, and B-Prolog.

The documentation for SWI-Prolog 5.10 (2010) even says "Tradition calls for .pl, but conflicts with Perl force the use of another extension on systems where extensions have global meaning, such as MS-Windows. On such systems .pro is the common alternative." https://www.swi-prolog.org/download/stable/doc/SWI-Prolog-5.....

Thing is, I can't confirm that tradition predates Perl 4 - call it 1993.

Turbo Prolog used ".pro" in the late 1980s - https://archive.org/details/bitsavers_borlandturOwnersHandbo... .

So did Prolog-2, according to the 1990 book at https://archive.org/details/advancedlogicpro0000unse/page/80... .

VPI Prolog from 1991 has an '".hc" file name extension (which stands for Horn Clause, the logic upon which the PROLOG language is based).'.

I even found one Prolog system from that era using ".txt", at https://archive.org/details/programminginpro0000cloc/page/26... .

The 1978 DEC10 Prolog user guide mentions PROG.PL as an example of a Prolog file with an extension. See 3.2 in https://userweb.fct.unl.pt/~lmp/publications/online-papers/U...

Maybe the 1975 Prolog I manual has more to say...

Good work!

I think it's even more explicit on p25 where it says "A module name can have the form either file.ext or just file. In the latter case the extension 'PL' is implied."