public function setLongdesc($longdesc)
    {
        if (Format::isUri($longdesc) === true || (is_string($longdesc) === true && empty($longdesc) === true)) {
            $this->longdesc = $longdesc;
        } else {
            $msg = "The 'longdesc' argument must be a valid URI, '" . $longdesc . "' given.";
            throw new InvalidArgumentException($msg);
        }
    }