def diff(a, b):
    """
    Return the difference between operations a and b.
    """
    if a is None:
        a = {}
    if b is None:
        b = {}
    keys = set(a.keys()).union(set(b.keys()))
    attributes = {}
    for k in keys:
        av, bv = a.get(k, None), b.get(k, None)
        if av != bv:
            attributes[k] = bv
    return attributes or None
