This is good, I had been searching for a decent little lisp like this for a while but most of the ones I found had garbage collector bugs (extremely difficult to locate and fix) or didn't properly implement tail calls. I ended up writing my own https://github.com/rain-1/single_cream but now there's a few solid ones!

Kilo lisp is just over 1000 lines of code, pretty impressive given what it does. It uses the parallel rails memory representation for pairs as taught in sicp. It even seems to be able to load and save "images"!

I'm not totally sure how the evaluator works. It uses these kinds of opcodes? enum { MHALT = 0, MEXPR, MLIST, MBETA, MRETN, MAPPL, MPRED, MNOTP, MSETQ, MPROG }; do they represent the control stack or something?