func (tw *Writer) writeRawHeader(blk *block, size int64, flag byte) error {
	if err := tw.Flush(); err != nil {
		return err
	}
	if _, err := tw.w.Write(blk[:]); err != nil {
		return err
	}
	if isHeaderOnlyType(flag) {
		size = 0
	}
	tw.curr = &regFileWriter{tw.w, size}
	tw.pad = blockPadding(size)
	return nil
}

// AddFS adds the files from fs.FS to the archive.