code: public static String updateMode(String existing, String update) throws IOException {
  	return toStringMode(updateMode(toOctalMode(existing), toOctalMode(update)));
      }
docs: update a string mode with a other (111+222=333 or 333+111=333 or 113+202=313)
  
  @param existing
  @param update
  @return
  @throws IOException