e</TT> otherwise.</P>
    </DD>

    <DT>
      <TT>choose <I>s</I></TT>
    </DT>
    <DD>
      <P>Returns <TT>SOME</TT> <TT><I>x</I></TT>, where <TT><I>x</I></TT> is an element
      of the set <TT><I>s</I></TT>. Returns <TT>NONE</TT> if <TT><I>s</I></TT>
      is the empty set. For red-black sets, <TT><I>x</I></TT> will be the
      smallest element in the set.</P>
    </DD>

    <DT>
      <TT>equal (<I>s1</I>, <I>s2</I>)</TT>
    </DT>
    <DD>
      <P>Returns <TT>true</TT> if <TT><I>s1</I></TT> and 