code: private void addSubSlide(final Node defaultSubSlide) {
  
          this.subSlides.add(model().getStepPosition(), defaultSubSlide);
          this.slideContent.getChildren().add(defaultSubSlide);
  
          StackPane.setAlignment(defaultSubSlide, Pos.CENTER);
  
      }
docs: Add a subslide node.
  
  @param defaultSubSlide the subslide node