I'm happy to see this moving forward.

Not convinved of the "PyIter_Send()" name, though. I don't consider this part of the iterator protocol. It's specific to generators and coroutines. Cython would probably guard its usage by "PyGen_CheckExact()" or "PyCoro_CheckExact()", and not use it for arbitrary iterators.

Since coroutines inherit the generator protocol more or less, I think "PyGen_Send()" is a more suitable name, better than "PyCoro_Send()".