public function setTtl($ttl)
    {
        /**
         * @see Zend_Validate_Int
         */
        require_once 'Zend/Validate/Int.php';
        $validate = new Zend_Validate_Int();
        if (!$validate->isValid($ttl)) {
            /**
             * @see Zend_Feed_Builder_Exception
             */
            require_once 'Zend/Feed/Builder/Exception.php';
            throw new Zend_Feed_Builder_Exception("you have to set an integer value to the ttl property");
        }
        $this->offsetSet('ttl', $ttl);
        return $this;
    }