code>a3</code>, if supplied, may be selected in
            any order from the table below.</li>

            <li>Otherwise, <code>Fn</code> must be [derived from]
            <code><a href="object.html#object-spec">object</a></code>, and
            <code>a1-a2</code>, if supplied, may be selcted in any order from
            the first two rows of the table below. To be useful,
            <code>fn</code> should be <a href=
            "http://www.python.org/doc/current/lib/built-in-funcs.html#l2h-6">
    