def difference(c1, c2):
    """Return the keys or items in c1 for which there is no key in c2.

        If c1 is None, then None is returned.  If c2 is None, then c1
        is returned.

        If neither c1 nor c2 is None, the output is a Set if c1 is a Set or
        TreeSet, and is a Bucket if c1 is a Bucket or BTree.
        """
