Expert systems were a hot topic in the 1970s and 1980s, but soon fell out of favour because they are too inflexible and not much more effective than hard-code if-statements.

Jess is a Java-based implementation of a traditional production-system style expert system shell. It gives you a LISP-like language for asserting facts and rules, and also allows close interaction with the immense Java library.

The book has a number of large examples that are explained in some detail.
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