<TT><I>s2</I></TT>
      are sets with equal elements, <TT>false</TT> otherwise.</P>
    </DD>

    <DT>
      <TT>subset (<I>s1</I>, <I>s2</I>)</TT>
    </DT>
    <DD>
      <P>Returns <TT>true</TT> if <TT><I>s1</I></TT> is a subset of
      <TT><I>s2</I></TT>, <TT>false</TT> otherwise.</P>
    </DD>

    <DT>
      <TT>disjoint (<I>s1</I>, <I>s2</I>)</TT>
    </DT>
    <DD>
      <P>Returns <TT>true</TT> if <TT><I>s1</I></TT> and <TT><I>s2</I></TT>
      are disjoint sets, <TT>false</TT> otherwise.</P>
  