code: @Override
    public boolean handleBack() {
      Screen currentScreen = currentScreen();
      if (currentScreen.handleBack()) {
        return true;
      } else {
        if (!atRoot()) {
          goBack();
          return true;
        } else {
          return false;
        }
      }
    }
docs: Handles back button press by user. Notifies current screen on back button press and allows screen to handle
  back press itself. If the current screen does not handle back press, and this Navigator has more than one screen,
  this Navigator will remove the current screen from its collection of screens and display the previous screen.
  If this Navigator only has one Screen when it receives a call to {@link #handleBack()} it will not handle the back
  button press.
  
  Call this method from within {@link Activity#onBackPressed()} of the activity associated with this Navigator.
  
  (Example usage) In the Activity class associated with this Navigator, put:
  <pre> <code>
  {@literal @}Override
  public void onBackPressed() {
  if (!navigator.handleBack()) {
  super.onBackPressed();
  }
  }
  </code> </pre>
  
  @return true if the Navigator consumed the back button click