[texhax] where are latex functions defined?

[Karl Berry]

    Thank you - that was helpful.  Now for a follow up question,
    where are the TeX functions defined?

plain.tex (`kpsewhich plain.tex`) and in the source code (tex.web +
tex.ch).  There are other change files for the other TeX variants
(pdftex, Omega, etc.).

HTH,
k