template <typename _Base> 

   class ClearableUndirGraphExtender : public _Base {

   public:

     typedef ClearableUndirGraphExtender Graph;

     typedef _Base Parent;

     typedef typename Parent::Node Node;

     typedef typename Parent::UndirEdge UndirEdge;

     typedef typename Parent::Edge Edge;

     void clear() {

       Parent::getObserverRegistry(Node()).clear();

       Parent::getObserverRegistry(UndirEdge()).clear();

       Parent::getObserverRegistry(Edge()).clear();

       Parent::clear();

     }

   };