The code later got translated into C++. However, we had a template language for describing pages that was basically Common Lisp with a few macros. These templates are stored on disk as s-expressions. They couldn't change the template language because some users had created their own. So the C++ version literally had to read Lisp programs off disk and execute them. I.e. it was a Lisp interpreter.

That's what I was talking about at the time when I described it somewhere as a new world record for Greenspun's tenth rule.

http://en.wikipedia.org/wiki/Greenspun%27s_Tenth_Rule