The index that the builtin function enumerate() returns
is a mere integer and will wrap back to a negative
number when it overflows. This seems to be by design
(unless I'm misunderstanding the point of
PyInt_FromLong), but is sufficiently surprising that
I'd suggest it's a bug anyway. I've attached a test
case - it takes a while to execute, but the results are
so deeply exciting that it's worth doing just for fun!