There’s more to changing Python’s grammar than editing Grammar/Grammar and Python/compile.c. This document aims to be a checklist of places that must also be fixed.
It is probably incomplete. If you see omissions, submit a bug or patch.
This document is not intended to be an instruction manual on Python grammar hacking, for several reasons.
People are getting this wrong all the time; it took well over a year before someone noticed that adding the floor division operator (//) broke the parser module.