[Python-ideas] Introduce collections.Reiterable

[Stephen J. Turnbull]

Nick Coghlan writes:

 > And these are exactly the cases that I am saying *deliberately*
 > fail the more formal check instituted in PEP 3119. The __getitem__
 > fallback is a backwards compatibility hack, not part of the formal
 > definition of an iterable.

I think that resolves my issue that __getitem__ is polymorphic, too.

That is, item access by integer index doesn't care about order (could
be a table of Goedel numbers) any more item access by arbitrary
hashable does, and __iter__ takes care of the cases where the
programmer does care about order.