protected function parseSort($sort)
    {
        $type = substr($sort, 0, 1);
        $key = substr($sort, 1);

        switch ($type) {
            case '+': $type = Sort::TYPE_ASC; break;
            case '-': $type = Sort::TYPE_DESC; break;
            default:
                throw new NoSuchSortException();
        }

        return new Sort($type, $key);
    }