code: public boolean back() {
          Fragment top = peek();
          if (top instanceof OnBackPressedHandlingFragment) {
              if (((OnBackPressedHandlingFragment)top).onBackPressed())
                  return true;
          }
          return pop();
      }
docs: Pops the top item if the stack.
  If the fragment implements {@link OnBackPressedHandlingFragment}, calls {@link OnBackPressedHandlingFragment#onBackPressed()} instead.
  If {@link OnBackPressedHandlingFragment#onBackPressed()} returns false the fragment gets popped.
  
  @return true if a fragment has been popped or if {@link OnBackPressedHandlingFragment#onBackPressed()} returned true;