TT>exists</TT>, <TT>find</TT> and <TT>filter</TT>.
      The index of each pair of elements
      (starting from 0) is passed to <TT><I>f</I></TT> as an additional
      argument. For <TT>appri</TT> and <TT>foldri</TT>, processing
      starts at the highest index. The functions <TT>findi</TT> and
      <TT>filteri</TT> return indices along with the corresponding pair
      of elements. The following equivalences hold:</P>
      <PRE>
        app <I>f</I> (<I>l1</I>, <I>l2</I>)        = appi (<I>f</I> o <I>