code: private boolean notifyOnPrevious(final int index) {
          boolean result = true;
  
          for (WizardListener listener : wizardListeners) {
              result &= listener.onPrevious(index, viewPagerAdapter.getItem(index));
          }
  
          return result;
      }
docs: Notifies all listeners, when the previous fragment is about to be shown.
  
  @param index
  The index of the fragment, which is about to be shown, as an {@link Integer} value
  @return True, if the fragment is allowed to be shown, false otherwise