template <typename _Base> 

   class ExtendableUndirGraphExtender : public _Base {

   public:

     typedef ExtendableUndirGraphExtender Graph;

     typedef _Base Parent;

     typedef typename Parent::Node Node;

     typedef typename Parent::Edge Edge;

     typedef typename Parent::UndirEdge UndirEdge;

     Node addNode() {

       Node node = Parent::addNode();

       Parent::getObserverRegistry(Node()).add(node);

       return node;

     }

     UndirEdge addEdge(const Node& from, const Node& to) {

       UndirEdge uedge = Parent::addEdge(from, to);

       Parent::getObserverRegistry(UndirEdge()).add(uedge);

       Edge edge_forward(uedge, true);

       Edge edge_backward(uedge, false);

       Parent::getObserverRegistry(Edge()).add(edge_forward);

       Parent::getObserverRegistry(Edge()).add(edge_backward);

       return uedge;

     }

   };