my $lsubscr;
  @cold = reverse @cold;
  @cnew = reverse @cnew; #we're gonna search these backwards
  for (0..$longer) { 
    if ($cold[$_] ne $cnew[$_]) {
        $lsubscr = -$_;
        last;
    }
  }
  #got pertinent subscripts, now insert span tags
  substr $old, $lsubscr, 0, $espan;
  substr $new, $lsubscr, 0, $espan;
  substr $old, $subscr, 0, $span;
  substr $new, $subscr, 0, $span;