This 1989 MSc thesis describes and includes the full source of AMUSED, a diagnostic assistance expert system written in Interlisp:
https://repository.rit.edu/theses/97
The program icon on page 45 of the PDF is worth a look for a chuckle:
https://repository.rit.edu/cgi/viewcontent.cgi?article=1100&context=theses#page=45