8 YAPES: Yet Another PROLOG Expert System T. B. Niblett

It provides inference and explanation facilities, and incorporates a novel form of plausible inference. YAPES is a specialized interpreter for logic programs. Figure 1 illustrates its top level structure. A PROLOG interpreter (or compiler) executes such programs consisting of sets of Horn clauses, a form of first-order logic. The YAPES system also executes such programs, as well as programs in an extended version of Horn clause logic which uses certainties as truth values, rather than just true and false.