template <typename _Base>

   class ExtendableUndirBipartiteGraphExtender : public _Base {

   public:

     typedef _Base Parent;

     typedef ExtendableUndirBipartiteGraphExtender Graph;

     typedef typename Parent::Node Node;

     typedef typename Parent::LowerNode LowerNode;

     typedef typename Parent::UpperNode UpperNode;

     typedef typename Parent::Edge Edge;

     typedef typename Parent::UndirEdge UndirEdge;

     Node addUpperNode() {

       Node node = Parent::addUpperNode();

       Parent::getNotifier(UpperNode()).add(node);

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

       return node;

     }

     Node addLowerNode() {

       Node node = Parent::addLowerNode();

       Parent::getNotifier(LowerNode()).add(node);

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

       return node;

     }

     UndirEdge addEdge(const Node& source, const Node& target) {

       UndirEdge undiredge = Parent::addEdge(source, target);

       Parent::getNotifier(UndirEdge()).add(undiredge);

       std::vector<Edge> edges;

       edges.push_back(Parent::direct(undiredge, true));

       edges.push_back(Parent::direct(undiredge, false));

       Parent::getNotifier(Edge()).add(edges);

       return undiredge;

     }

   };