ession is equivalent to:</P>
      <PRE>
         List.mapPartial f (zip (l1, l2)).</PRE>
    </DD>

    <DT>
      <TT>find <I>f</I> (<I>l1</I>, <I>l2</I>)</TT>
    </DT>
    <DD>
      <P>Applies <TT><I>f</I></TT> to each pair <TT>(<I>x1</I>,<I>x2</I>)</TT>
      of elements of the lists, from left to right, until
      <TT><I>f</I>(<I>x1</I>,<I>x2</I>)</TT>
      evaluates to <TT>true</TT>; returns <TT>SOME(<I>x1</I>,<I>x2</I>)</TT> if
      such a pair exists and <TT>NONE</TT> otherwise. If the lists ar