public function addEnclosure($url, $type = '', $length = '')
    {
        if (!$this->offsetExists('enclosure')) {
            $enclosure = array();
        } else {
            $enclosure = $this->offsetGet('enclosure');
        }
        $enclosure[] = array('url' => $url,
                             'type' => $type,
                             'length' => $length);
        $this->offsetSet('enclosure', $enclosure);
        return $this;
    }