code: @CommandArgument
    public void rmuser(
        @OptionArgument("username") String username) {
  
      if (users.containsKey(username)) {
        users.remove(username);
      } else {
        System.err.println(String.format("User '%s' does not exist", username));
      }
    }
docs: Deletes a user.
  @param username The username of the user to remove.