  </DD>

    <DT>
      <TT>compare (<I>s1</I>, <I>s2</I>)</TT>
    </DT>
    <DD>
      <P>Returns <TT>EQUAL</TT> if <TT><I>s1</I></TT> and <TT><I>s2</I></TT>
      are equal sets, <TT>LESS</TT> if <TT><I>s1</I></TT> is a subset of
      <TT><I>s2</I></TT>, and <TT>GREATER</TT> if <TT><I>s2</I></TT> is a
      subset of <TT><I>s1</I></TT>. Otherwise it raises <TT>Unordered</TT>.</P>
    </DD>

    <DT>
      <TT>app <I>f</I> <I>s</I></TT>
    </DT>
    <DD>
      <P>Applies <TT><I>f</I></TT> to each elemen