private function _add_sorters($qe, $thenmap)
 {
     foreach ($qe->get_float("sort", array()) as $s) {
         if ($s = ListSorter::parse_sorter($s)) {
             $s->thenmap = $thenmap;
             $this->sorters[] = $s;
         }
     }
     if (!$qe->get_float("sort") && $qe->type === "pn") {
         $pn = array_diff($qe->value[0], $qe->value[1]);
         $s = ListSorter::make_field(new NumericOrderPaperColumn(array_flip($pn)));
         $s->thenmap = $thenmap;
         $this->sorters[] = $s;
     }
 }