/TT> return equal results <TT><I>x</I></TT>. Equivalent to
      <TT>fromList(List.mapPartial <I>f</I> (toList <I>s</I>))</TT>.</P>
    </DD>

    <DT>
      <TT>fold <I>f</I> <I>a</I> <I>s</I></TT>
    </DT>
    <DD>
      <P>Sequentially applies <TT><I>f</I></TT> to the pair of a set item and
      the result of the previous application, starting with initial value
      <TT><I>a</I></TT>.
      For red-black trees, folding is performed in increasing order.
      Equivalent to <TT>List.foldl <I>f</I> <I>a