class MenuLinkTreeElement

class MenuLinkTreeElement {
    
    /**
     * The menu link for this element in a menu link tree.
     *
     * @var \Drupal\Core\Menu\MenuLinkInterface
     */
    public $link;
    
    /**
     * The subtree of this element in the menu link tree (this link's children).
     *
     * (Children of a link are only loaded if a link is marked as "expanded" by
     * the query.)
     *
     * @var \Drupal\Core\Menu\MenuLinkTreeElement[]
     */
    public $subtree;
    
    /**
     * The depth of this link relative to the root of the tree.
     *
     * @var int
     */
    public $depth;
    
    /**
     * Whether this link has any children at all.
     *
     * @var bool
     */
    public $hasChildren;
    
    /**
     * Whether this link is in the active trail.
     *
     * @var bool
     */
    public $inActiveTrail;
    
    /**
     * Whether this link is accessible by the current user.
     *
     * If the value is NULL the access was not determined yet, if an access result
     * object, it was determined already.
     *
     * @var \Drupal\Core\Access\AccessResultInterface|null
     */
    public $access;
    
    /**
     * Additional options for this link.
     *
     * This is merged (\Drupal\Component\Utility\NestedArray::mergeDeep()) with
     * \Drupal\Core\Menu\MenuLinkInterface::getOptions(), to allow menu link tree
     * manipulators to add or override link options.
     *
     * @var string[]
     */
    public $options = [];
    
    /**
     * Constructs a new \Drupal\Core\Menu\MenuLinkTreeElement.
     *
     * @param \Drupal\Core\Menu\MenuLinkInterface $link
     *   The menu link for this element in the menu link tree.
     * @param bool $has_children
     *   A flag as to whether this element has children even if they are not
     *   included in the tree (i.e. this may be TRUE even if $subtree is empty).
     * @param int $depth
     *   The depth of this element relative to the tree root.
     * @param bool $in_active_trail
     *   A flag as to whether this link was included in the list of active trail
     *   IDs used to build the tree.
     * @param \Drupal\Core\Menu\MenuLinkTreeElement[] $subtree
     *   The children of this element in the menu link tree.
     */
    public function __construct(MenuLinkInterface $link, $has_children, $depth, $in_active_trail, array $subtree) {
        // Essential properties.
        $this->link = $link;
        $this->hasChildren = $has_children;
        $this->depth = $depth;
        $this->subtree = $subtree;
        $this->inActiveTrail = $in_active_trail;
    }
    
    /**
     * Counts all menu links in the current subtree.
     *
     * @return int
     *   The number of menu links in this subtree (one plus the number of menu
     *   links in all descendants).
     */
    public function count() {
        $sum = function ($carry, MenuLinkTreeElement $element) {
            return $carry + $element->count();
        };
        return 1 + array_reduce($this->subtree, $sum);
    }

}