To count number of quadruplets with sum = 0

[Paul Rubin]

Steven Bethard <steven.bethard at gmail.com> writes:
> > According to a post by Raymond Hettinger it's faster to use that
> > iterator instead of `int`.
> Yep. It's because the .next() method takes no arguments, while int()
> takes varargs because you can do:: ...

Heh, good point.  Might be worth putting a special hack in defaultdict
to recognize the common case of defaultdict(int).