public function setType($type)
    {
        // check type
        if (!in_array($type, array(self::AUTO, self::ISBN10, self::ISBN13))) {
            /**
             * @see Zend_Validate_Exception
             */

            throw new Zend_Validate_Exception('Invalid ISBN type');
        }

        $this->_type = $type;
        return $this;
    }