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.
That applies to TeX *ports*. And there is a torture test to pass.
--T