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