code: public static void setPersistentMenu(List<Button> buttons) {
  		if (buttons == null || buttons.isEmpty() || buttons.size() > 5) {
  			logger.error("FbBotMill validation error: Persistent Menu Buttons can't be null or empty and must be less than 5!");
  			return;
  		}
  		CallToActionsRequest request = new CallToActionsRequest(
  				ThreadState.EXISTING_THREAD, buttons);
  		FbBotMillNetworkController.postThreadSetting(request);
  	}
docs: Sets a Persistent Menu of buttons which is always available to the user.
  This menu should contain top-level actions that users can enact at any
  point. Having a persistent menu easily communicates the basic
  capabilities of your bot for first-time and returning users. The menu can
  be invoked by a user, by tapping on the 3-caret icon on the left of the
  composer.
  
  @param buttons
  a list of {@link Button} (max 5 elements) to use as menu. The
  buttons can only be {@link PostbackButton} or
  {@link WebUrlButton}. Phone buttons are not supported.
  @see <a href=
  "https://developers.facebook.com/docs/messenger-platform/thread-settings/persistent-menu"
  >Facebook's Persistent Menu Documentation</a>