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Totally right.

I have one more concern:

>>> import sys
>>> (sys.maxint+1).__index__()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
OverflowError: long int too large to convert to int
>>> 

whiile I agree that the C API ought to silently truncate
values to fit within the boundaries established by ssize_t,
I think that when calling __index__() from Python, it should
be allowed to return an unbounded value.

Anyway, I haven't had time to whip up code to implement
that, but here's a new patch that adds the test to ISINDEX()
-- thanks for catching that!