template <typename _Base> 

   class AlterableEdgeSetExtender : public _Base {

   public:

     typedef AlterableEdgeSetExtender Graph;

     typedef _Base Parent;

     typedef typename Parent::Edge Edge;

     /// The edge observer registry.

     typedef AlterationNotifier<Edge> EdgeNotifier;

   protected:

     mutable EdgeNotifier edge_notifier;

   public:

     /// \brief Gives back the edge alteration notifier.

     ///

     /// Gives back the edge alteration notifier.

     EdgeNotifier& getNotifier(Edge) const {

       return edge_notifier;

     }

     ~AlterableEdgeSetExtender() {

       edge_notifier.clear();

     }

   };

   template <typename _Base> 

   class AlterableUndirEdgeSetExtender

     : public AlterableEdgeSetExtender<_Base> {

   public:

     typedef AlterableUndirEdgeSetExtender Graph;

     typedef AlterableEdgeSetExtender<_Base> Parent;

     typedef typename Parent::UndirEdge UndirEdge;

     typedef AlterationNotifier<UndirEdge> UndirEdgeNotifier;

   protected:

     mutable UndirEdgeNotifier undir_edge_notifier;

   public:

     using Parent::getNotifier;

     UndirEdgeNotifier& getNotifier(UndirEdge) const {

       return undir_edge_notifier;

     }

     ~AlterableUndirEdgeSetExtender() {

       undir_edge_notifier.clear();

     }

   };