protected function addValidators()
    {
        foreach (config('countries.validation.rules') as $ruleName => $countryAttribute) {
            if (is_int($ruleName)) {
                $ruleName = $countryAttribute;
            }

            Validator::extend($ruleName, function ($attribute, $value) use ($countryAttribute) {
                return ! CountriesFacade::where($countryAttribute, $value)->isEmpty();
            }, 'The :attribute must be a valid '.$ruleName.'.');
        }
    }