public function isRequired() : Group
    {
        foreach ((array)$this as $el) {
            if (!is_object($el)) {
                continue;
            }
            $el->getElement()->attribute('required', 1);
        }
        return $this->addTest('required', function ($value) {
            foreach ($value as $option) {
                if ($option->getElement() instanceof Radio
                    && $option->getElement()->checked()
                ) {
                    return true;
                }
            }
            return false;
        });
    }