discovering whether package is installed

Anne Ogborn <[email protected]> Tue, 8 Apr 2014 16:14:44 -0700 (PDT)
Newsgroups gmane.comp.ai.prolog.swi
Message-ID <[email protected]>
Writing some code that will be deployed, and depends on several packages. I'm writing a preflight predicate that checks that everything's happy prior to startup.

I can't seem to find a way to test if a pack is installed. Suggestions?