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An alternative fix for this would be not to invoke
filter{tuple,string,unicode} on instances of subclasses of
tuple, str and unicode.

This would fix this bug because you have to be using a
subclass of one of these types to exploit the preallocation.

As a side-effect, this would also resolve the issue I raised
in bug #1517509 concerning filter()'s treatment of these
subtypes re: the iterator protocol.