/// This map returns the out-degree of a node. Once it is constructed,

   /// This map returns the out-degree of a node. Once it is constructed,

   /// the degrees are stored in a standard NodeMap, so each query is done

   /// the degrees are stored in a standard NodeMap, so each query is done

   /// in constant time. On the other hand, the values are updated automatically

   /// in constant time. On the other hand, the values are updated automatically

   /// whenever the graph changes.

   /// whenever the graph changes.

   ///

   ///

   template <typename _Graph>

   template <typename _Graph>

   class OutDegMap  

   class OutDegMap  

     : protected AlterationNotifier<typename _Graph::Edge>::ObserverBase {

     : protected AlterationNotifier<typename _Graph::Edge>::ObserverBase {