template <typename _Base> 

   class ErasableUndirGraphExtender : public _Base {

   public:

     typedef ErasableUndirGraphExtender Graph;

     typedef _Base Parent;

     typedef typename Parent::Node Node;

     typedef typename Parent::UndirEdge UndirEdge;

     typedef typename Parent::Edge Edge;

     void erase(const Node& node) {

       Edge edge;

       Parent::firstOut(edge, node);

       while (edge != INVALID ) {

 	erase(edge);

 	Parent::firstOut(edge, node);

       } 

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

       Parent::erase(node);

     }

     void erase(const UndirEdge& uedge) {

       Parent::getObserverRegistry(Edge()).erase(Edge(uedge,true));

       Parent::getObserverRegistry(Edge()).erase(Edge(uedge,false));

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

       Parent::erase(uedge);

     }

   };