def compareLists(c1,c2):
    """
    Compare a pair of lists, returning None if the lists are identical, 
    or a pair of lists containing:
    (1) elements of first list not in second, and
    (2) elements of second list not in first list.
    """
    c1 = c1 or []
    c2 = c2 or []
    c1d = []
    c2d = []
    for c in c1: 
        if not (c in c2): c1d.append(c)
    for c in c2: 
        if not (c in c1): c2d.append(c)
    if c1d or c2d: return (c1d,c2d)
    return None