Support for new items in set type

[Raymond Hettinger]

[Alex Martelli]
> In your shoes, I would write a class whose instances hold three sets:
> -- the "master set" is what you originally read from the file
> -- the "added set" is the set of things you've added since then
> -- the "deleted set" is the set of things you've deleted since them

FWIW, I've posted a trial implementation in the Python cookbook:
http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/511496


Raymond Hettinger