def transform(a, b, priority=True):
    """
    Return the transformation from operation a to b.

    If ``priority`` is falsey [default=True] then just return b.
    """
    if a is None:
        a = {}
    if b is None:
        b = {}
    if not priority:
        return b or None
    attributes = {}
    for k, v in b.items():
        if k not in a:
            attributes[k] = v
    return attributes or None
