t (<I>s1</I>, <I>s2</I>)</TT>
    </DT>
    <DD>
      <P>Returns the intersection of sets <TT><I>s1</I></TT> and
      <TT><I>s1</I></TT>.</P>
    </DD>

    <DT>
      <TT>difference (<I>s1</I>, <I>s2</I>)</TT>
    </DT>
    <DD>
      <P>Returns the difference of sets <TT><I>s1</I></TT> and
      <TT><I>s1</I></TT>, i.e. the sets of elements that are in
      <TT><I>s1</I></TT> but not in <TT><I>s2</I></TT>.</P>
    </DD>

    <DT>
      <TT>size <I>s</I></TT>
    </DT>
    <DD>
      <P>Returns the card