public function createSubnavMenu($menuName, $classes = 'nav nav-tabs nav-stacked')
    {
        $menu = $this->factory->createItem('root');

        if ($classes) {
            $menu->setChildrenAttribute('class', $classes);
        }

        /** @var $mainMenu Menu */
        $menuIterator = $this->getSubMenu($menuName, 1);

        if (!$menuIterator) {
            return $menu;
        }

        $startDepth = 2;

        $menu = $this->createMenu($menu, $menuIterator, $startDepth);
        $this->showOnlyCurrentChildren($menu);
        $this->setRecursiveAttribute($menu, ['class' => 'nav nav-list']);

        return $menu;
    }