protected function build_thread($items, $level = 1, &$pos = 0)
    {
        $result = array();

        for ($len=count($items); $pos < $len; $pos++) {
            list($lv, $id) = explode(self::SEPARATOR_LEVEL, $items[$pos]);
            if ($level == $lv) {
                $pos++;
                $result[$id] = $this->build_thread($items, $level+1, $pos);
            }
            else {
                $pos--;
                break;
            }
        }

        return $result;
    }