An expert system can be defined as a computer program which uses artificial intelligence techniques to perform or to guide a task that a human expert can do. Feigenbaum (1977) shifts the amphasis from techniques and formalisms to the knowledge that an expert system contains. The expert knowledge seems to be a necessary and a nearly sufficient condition for developing an expert system (Hays-Roth et all 1983, p. 7). It seems to be clear that the acquisition of this knowledge as a technique (or is it still an art?) is an important field of study within the knowledge-engineering process.