@Override
    public int getIndexOfChild(Object parent, Object child) {
	if (!(parent instanceof ZVNode)) {
	    throw new IllegalArgumentException("parent must be a ZVNode");
	}
	if (!(child instanceof ZVNode)) {
	    throw new IllegalArgumentException("child must be a ZVNode");
	}
	return this.model.getChildren((ZVNode) parent).indexOf(child);
    }