I>s</I>, <I>x</I>)</TT>
    </DT>
    <DD>
      <P>Returns the set <TT><I>s</I></TT> with element
      <TT><I>x</I></TT> removed. In the first form, if no element equal to
      <TT><I>x</I></TT> is contained in <TT><I>s</I></TT>, then the set is
      returned unchanged. In the second form, <TT>Unknown</TT>
      <TT><I>x</I></TT> will be raised. In the third form, <TT><I>f</I></TT> is
      applied to <TT><I>x</I></TT> before the set is returned unchanged.
      The following equivalences hold:</P>
    