def difference(a, b):
    """
    Returns the symmetric difference of sets 'a' and 'b'.

    In plain english:
        Removes all items that occur in both 'a' and 'b'

    Difference is actually set_a.symmetric_difference(set_b), not 
    set.difference(). See 'minus' for set.difference().
    """
    return a.symmetric_difference(b)