protected function key(array $notice): string
    {
        $hashable_notice = $this->normalize($notice);
        unset($hashable_notice['_insertion_time']);
        unset($hashable_notice['_recurrences']);
        unset($hashable_notice['_last_recur_dismiss_time']);

        if ($hashable_notice['id']) {
            return $hashable_notice['id']; // Use as key.
        }
        return $this->c::sha256KeyedHash(serialize($hashable_notice));
    }