public:

Destructor deletes just the node, not the private_data pointer
as the data element could be on more than one list.  Takes O(1) time.
                         ~Node();

Getters for traversing the list.  Each method takes O(1) time.
   Node<X>*        get_next();
   Node<X>*        get_previous();
   List<X>*        get_owner();

Getters and setters for the private_data pointer.
Each method takes O(1) time.
   X*              get_data();
   void            set_data(X* x);

Const versions of the above methods. Each method takes O(1) time.
   const Node<X>*  get_next_const() const;
   const Node<X>*  get_previous_const() const;
   const List<X>*  get_owner_const() const;
   const X*        get_data_const() const;

Methods for adding to the list.  Each method takes O(1) time.
   void            add_before_current(X* x);
   void            add_after_current(X* x);

Eliminates get_data() in client code. Each method takes O(1) time.
   void            add_before_current(Node<X>* n);
   void            add_after_current(Node<X>* n);

private:

Pointer to the data object.
   X*              private_data;

Pointer to the next node of the list.
   Node<X>*        private_next;

Pointer to the previous node of the list.
   Node<X>*        private_previous;

Pointer to the list that the node belongs to.
   List<X>*        private_owner;

Private constructor means that
only indirect means of construction are possible.  Takes O(1) time.
                   Node();

Disallow passing and returning by value.
   Node&           operator = (const Node& n);
                         Node(const Node& n);