private function _index_tags()
    {
        $s   = $nix   = $ix   = self::$_ar_;
        $ids = $this->ids;
        foreach ($this->tags as $id => $n) {
            // if(!isset($ix[$n])) $ix[$n] = array();
            $ix[$n][$id] = $ids[$id];
        }
        foreach ($ix as $n => $v) {
            foreach ($v as $id => $e) {
                $this->tags[$id] = $n;
            }

            if (isset($nix[$n])) {
                continue;
            }

            $_n = strtolower($n);
            if (isset($nix[$_n])) {
                foreach ($v as $id => $e) {
                    $nix[$_n][$id] = $e;
                }

                $s[] = $_n;
            } else {
                $nix[$_n] = $v;
            }
        }
        foreach ($s as $_n) {
            asort($nix[$_n]);
        }

        return $this->tag_idx = $nix;
    }