public function listSubscribe($options) {
        if (!$options instanceof Garp_Util_Configuration) {
            $options = new Garp_Util_Configuration($options);
        }
        $options->obligate('email_address')
                ->obligate('id')
                ->setDefault('merge_vars', array('LNAME' => '', 'FNAME' => ''))
                ;
        $options['method'] = 'listSubscribe';
        return $this->_send((array)$options);
    }