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Math is also handled poorly in Prolog.  Consider the following program:

  convert(Celsius, Fahrenheit) :-
       Celsius is (Fahrenheit - 32) * 5 / 9.

You can then issue the query C<convert(Celsuis, 32)> and it will dutifully
report that the celsius value is zero.  However, C<convert(0, 32)> fails.  This
is because Prolog is not able to solve the right hand side of the equation
unless C<Fahrenheit> has a value at the time that Prolog starts examining the 
equation.  One simplistic way of getting around this limitation is with multiple
predicates and testing which argument is an unbound variable:

 convert(Celsius, Fahrenheit) :-
     var(Celsius),
     not(var(Fahrenheit)),
     Celsius is (Fahrenheit - 32) * 5 / 9.

 convert(Celsius, Fahrenheit) :-
     var(Fahrenheit),
     not(var(Celsius)),
     Fahrenheit is (Celsius * (9/5)) + 32.

This has a variety of problems, not the least of which is that it offers no
advantages over imperative programming.

ISO-Prolog does not define it, but many Prolog implementations support
constraints and these, though beyond the scope of this article, can get around
this and other issues.  They can allow "logical" math and ensure that
"impossible" search branches are never explored.  This can help to alleviate
many of the aforementioned concerns.

=head1 Conclusion

We've barely scratched the surface of what the Prolog language can do.  The
language has many detractors and some criticisms are quite valid.  However, the
language's simplicity and ease-of-use has kept it around.  It's an excellent
starting point for understanding logic programming and exploration of AI.  In
fact, many common uses for Prolog are heavily used in the AI arena:

=over 4

=item * Agent-based programming

=item * Expert Systems

=item * Fraud prevention (via inductive logic programming)

=item * Natural language processing

=item * Rules-based systems

=item * Scheduling systems

=item * And much more (it was even embedded in Windows NT) 

=back

At the present time, I would not recommend C<AI::Prolog> for production work.
Attempts to bring logic programming to Perl are still relatively recent and no
module (to this author's knowledge) is currently ready for widespread use.  The
C<AI::Prolog> distribution has a number of sample programs in the C<examples/>
directory and two complete, albeit small, games in the C<data/> directory.

=head1 References

=head2 Online Resources

=over 4

=item Adventure in Prolog

http://amzi.com/AdventureInProlog/index.htm

I highly recommend Amzi! Prolog's free online book "Adventure in Prolog."  It's
clearly written and by the time you're done, you've built Prolog programs for
an adventure game, a genealogical database, a customer order inventory
application and a simple expert system.

=item Building Expert Systems in Prolog

http://amzi.com/ExpertSystemsInProlog/xsipfrtop.htm

This free online book, also by Amzi!, walks you through building expert
systems.  It includes expert systems for solving Rubik's cube, tax preparation,
car diagnostics and many other examples.

=item Databases Vs AI

http://lsdis.cs.uga.edu/~cartic/SemWeb/DatabasesAI.ppt

This Powerpoint presentation discusses AI in Prolog and compares it to SQL
databases.

=item W-Prolog

http://goanna.cs.rmit.edu.au/~winikoff/wp/

Dr. Michael Winikoff kindly gave me permission to port this Java application to
Perl.  This formed the basis of the earliest versions.

=item X-Prolog

http://www.iro.umontreal.ca/~vaucher/XProlog/

Jean Vaucher, Ph.D., allowed me to borrow from X-Prolog for the first
implementation of math in C<AI::Prolog>.  Currently, C<AI::Prolog> is a hybrid
of these two applications, but it has evolved in a much different direction.

=back

=head2 Books

=over 4

=item Programming in Prolog

http://portal.acm.org/citation.cfm?id=39071

This book is sometimes referred to simply as "Clocksin/Mellish".  First
published in the early 80s, this is the book that brought Prolog into the
mainstream.