code: @CommandArgument
    public void save(
        @OptionArgument("filename") String filename) {
  
      try {
        PrintStream out = new PrintStream(new FileOutputStream(filename));
  
        for (User user : users.values()) {
          out.println(user);
        }
      } catch (IOException e) {
        e.printStackTrace();
      }
  
    }
docs: Saves a users file.
  @param filename The filename of the file to save.