When trying to remove a set from a set, the KeyError that is raised is
confusing:

Python 2.6 (r26:66714, Oct  7 2008, 13:23:57)
[GCC 4.2.3 (Ubuntu 4.2.3-2ubuntu7)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> s = set([frozenset([1, 2]), frozenset([2, 3])])
>>> s.remove(set([3, 4]))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: frozenset([])


I understand that s.remove(set(...)) turns the set into a frozenset, but
I was expecting this converted frozenset to be attached to the KeyError,
not an empty set.