New tests also added.

I also made the comments in line with the builtin Grammar/Grammar. PEP 306 was
withdrawn, Kees Blom's railroad program has been lost to the sands of time for
at least 16 years now (I found a python-dev post from people looking for it).
(cherry picked from commit b51f5de7)
Co-authored-by: Łukasz Langa <lukasz@langa.pl>