protected function _sort()
        {
            if ($this->_dirtyIndex) {
                $newIndex = array();
                $index = 0;

                foreach ($this->_pages as $hash => $page) {
                    $order = $page->getOrder();
                    if ($order === null) {
                        $newIndex[$hash] = $index;
                        $index++;
                    } else {
                        $newIndex[$hash] = $order;
                    }
                }

                asort($newIndex);

                $this->_index = $newIndex;
                $this->_dirtyIndex = false;
            }
        }