In the User Focus section, you learned how to create predicate logic rules for a knowledge base. The syntax for those rules is modeled on Prolog, a programming language popular for developing expert systems. For more information on the syntax of these rules, use your favorite Web search engine to search for "Prolog." A good source of information is the Amzi! site at www.amzi.com. At this site, you can click the "Search & Site Advisor" link to use an expert system that interprets information you provide to determine which articles about Prolog it should recommend. You'll especially like two of the articles at this site. The first, "White Paper on Rules, Prolog, and Logic Server Technology" (www.amzi.com/articles/prolog_whitepaper.htm), provides a quick overview and examples of how to use rules. A second article, "Exploring Prolog" (www.amzi.com/articles/prolog_fun.htm), describes how to use Prolog to write an adventure game. The classic book on Prolog is Programming in Prolog, fourth edition by Clocksin and Mellish (Springer-Verlag, 1994). Another popular book is The Art of Prolog: Advanced Programming Techniques by Leon Sterling and Ehud Shapiro (editor) (MIT Press, 1994).