http://www.cs.utexas.edu/users/novak/tmycin.html
TMYCIN is written in Common Lisp and is fairly small (11 pages of code).
It should run under any implementation of Common Lisp; it will run
under the student version of Gold Hill Lisp with the addition of the
file
Two example expert systems are furnished: a very small set of rules for identifying rocks, and a small set of rules for identifying Texas snakes.