TeX is one of those systems (like Perl, etc) which has
one
implementation. Can you _imagine_ the task of building a TeX-compatible
compiler whose compliance test were typesetting the TeXbook... :-X
I beleive there _are_ a set of compliance tests for a TeX implementation,
point being that all 'TeXs' have to split lines, etc in exactly the same
way under all conmditions. This means handling round errors correctly and
things like that.
-tony