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New Features
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- `ImmutableSet` now implements the same methods as the standard library `frozenset`. Added the `issubset`, `issuperset`, and `symmetric_difference` convenience methods that wrap `<=`, `>=`, and `^`, respectively, and the shallow `copy` method. (`61 <https://github.com/isi-vista/immutablecollections/issues/61>`_)
Bug Fixes
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- Slicing of `ImmutableSet`s that only contain a single element is implemented. (`23 <https://github.com/isi-vista/immutablecollections/issues/23>`_)