public static function getSubPathOf(array $subDir, array $parentDir) : string
    {
        if (!self::isSubDirectoryOf($subDir, $parentDir)) {
            throw new \RuntimeException('Invalid sub directory');
        }

        $path = [];
        foreach (array_slice($subDir, count($parentDir)) as $dir) {
            $path[] = $dir;
        }
        return implode(DIRECTORY_SEPARATOR, $path);
    }