Incidentally, it is common to store Prolog code in files with a .pl suffix. It’s an indication of what the file contains (namely Prolog code) and with some Prolog implementations you don’t actually have to type in the .pl suffix when you consult a file. Nice — but there is a drawback. Files containing Perl scripts usually have a .pl suffix too, and nowadays there are a lot of Perl scripts in use, so this can cause confusion. C’est la vie.