[issue8425] a -= b should be fast if a is a small set and b is a large set

[Alexander Belopolsky]

Alexander Belopolsky <belopolsky at users.sourceforge.net> added the comment:

On the second thought, it is possible to preserve set identity and avoid iteration over a large set.   See issue8425a.diff attached.

It looks like the unit tests don't check identity preservation.  I will submit additional tests shortly.

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Added file: http://bugs.python.org/file17283/issue8425a.diff

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