public function write_setting($data) {
        if (!is_array($data)) {
            return '';
        }
        unset($data['xxxxx']);
        $result = 0;
        foreach($data as $index) {
            $result |= 1 << $index;
        }
        return ($this->config_write($this->name, $result) ? '' : get_string('errorsetting', 'admin'));
    }