Mark:
> `is` is not well defined except for a small set of values, so the docs for `Py_Is` would have to so vague as to be worthless, IMO.

I don't propose to change the "is" operator semantic: Py_Is(x, y) is C would behave *exaclty* as "x is y" in Python.

Check my implementation: the IS_OP bytecode uses directly Py_Is().