public function add($file_name_or_array, bool $flatten_root_folder = false): ZipManager {

        try {

            foreach ( $this->zip_archives as $archive ) {
                $archive->add($file_name_or_array, $flatten_root_folder);
            }
            return $this;

        } catch (ZipException $ze) {
            throw $ze;
        }

    }