When I compare the books on expert systems in my library with the production expert systems I know of, I note that
there are few good books on building expert systems in Prolog. Of course, the set of actual production systems is a little
small for a valid statistical sample, at least at the time and place of this writing – here in Germany, and in the first days
of 1989. But there are at least some systems I have seen running in real life commercial and industrial environments,
and not only at trade shows.