public function group($spec)
    {
        if (!is_array($spec)) {
            $spec = array($spec);
        }

        foreach ($spec as $val) {
            if (preg_match('/\(.*\)/', (string) $val)) {
                $val = new Zend_Db_Expr($val);
            }
            $this->_parts[self::GROUP][] = $val;
        }

        return $this;
    }