public static function load($id): bool
    {
        $loaded = false;
        if (isset(static::$files[$id])) {
            $loaded = static::$files[$id] === true;

            if (! $loaded) {
                assert(file_exists(static::$files[$id]));

                include static::$files[$id];
                $loaded = static::$files[$id] = true;
            }
        }
        return $loaded;
    }