The inference engine of a typical expert system shell is part of a so-called consultation
system. The consultation system further comprises a user interface for the interaction with the
user, mostly in the form of question-answering sessions. Furthermore, the user of the expert
system and the knowledge engineer are provided with a variety of facilities for investigating the
contents of the knowledge base and the reasoning behaviour of the system. The explanation
facilities offer the possibility to ask at any moment during the course of the consultation of
the knowledge base how certain conclusions were arrived at, why a specific question is asked,
or why other conclusions on the contrary have not been drawn. By using the trace facilities
available in the consultation system, the reasoning behaviour of the system can be followed
one inference step at a time during the consultation. It turns out that most of these facilities
are often more valuable to the knowledge engineer, who applies them mainly for debugging
purposes, than to the final user of the system. Chapter 6 deals with these facilities. Figure 1.1
shows the more or less characteristic architecture of an expert system, built using an expert
system shell.