For operations that test membership in a set, Python coerces sets and subclasses of set into a temporary frozenset before testing membership.

For example, this works:

>>> set() in set([frozenset()])
True

Although the set() is not hashable itself, Python creates a temporary frozenset() out of the set().  It should do the same for user-types derived from collections.Set, so they can inter-operate in the same way.

In setobject.c, the following methods behave in the described manner: set_contains, set_remove, and set_discard.