Sometimes, a non-removing pop() is needed. In current Python versions,
it can achieved by one of the following ways:

1. 
x = some_set.pop()
some_set.add(x)

2. 
for x in some_set: 
        break

3.
x = iter(some_set).next()

More native and clean would, however, be 
some_set.get()

The attached patch does this for set(). If this is accepted by the
community, frozenset should be extended as well.