However, the theory has not been implemented for reasoning in expert systems due to.its difficulty dealing with uncertain rules. More recently, several extenstions to the theory has been proposed to overcome this difficulty [Yen, 1986a] [Liu, 1986]. Based on Yen's extended DS theory, we have implemented a prototype expert system, named GERTIS (General Evidential Reasoning Tool for Intelligent Systems), that diagnoses rheumatoid arthritis. We chose unspecified polyarthritis as the area of our medical consultation system because the diagnoses form a disease hierarchy, which fits Dempster-Shafer based reasoning best. GERTIS uses the knowledge base of OADIAG-2, a medical expert system developed by Peter Adlassnig [Adlassnig, 1985a,b]. Through the use of OADIAG-2's knowledge base, relevant evidence and rules have been already identified for the area of arthritis. In order to suit the needs of our model, however, the rules of OADIAG-2 were modified and reorganized.

The satisfiability problem for monadic infinite-valued Gödel logic is known to be undecidable. We identify a fragment of this logic extended with strong negation whose satisfiability is not only decidable but it is decidable within classical logic. We use this fragment to formalize the rules of CADIAG-2, a well performing fuzzy expert system assisting in the differential diagnosis in internal medicine. A (classical) satisfiability check of the resulting formulas allowed the detection of some errors in the rules of the system.