public function setBlock($block)
    {
        $block = strtolower($block);
        if (!in_array($block, array('yes', 'no'))) {
            /**
             * @see Zend_Feed_Builder_Exception
             */
            require_once 'Zend/Feed/Builder/Exception.php';
            throw new Zend_Feed_Builder_Exception("you have to set yes or no to the itunes block property");
        }
        $this->offsetSet('block', $block);
        return $this;
    }