I don't know if people worrying about typesetting of math
have been using this, ignoring this for some reason, or
are unaware of it. But there is a description, listing
etc. of programs to interpret TeX, written in Lisp.
It's been around since 1994 or so; I just came across
a copy of it while I was cleaning up some file
directories. I checked to see that it was still
around on the www. It seems so.
ftp://ftp.tu-darmstadt.de/pub/tex/src/etls/
RJF