/// \brief An empty non-static undirected graph class.

     ///    

     /// This class provides everything that \ref BpUGraph does.

     /// Additionally it enables building graphs from scratch.

     class ExtendableBpUGraph : public BpUGraph {

     public:

       /// \brief Add a new ANode to the graph.

       ///

       /// Add a new ANode to the graph.

       /// \return the new node.

       Node addANode();

       /// \brief Add a new ANode to the graph.

       ///

       /// Add a new ANode to the graph.

       /// \return the new node.

       Node addBNode();

       /// \brief Add a new undirected edge to the graph.

       ///

       /// Add a new undirected edge to the graph. One of the nodes

       /// should be ANode and the other should be BNode.

       /// \pre The nodes are not in the same nodeset.

       /// \return the new edge.

       UEdge addEdge(const Node& from, const Node& to);

       /// \brief Resets the graph.

       ///

       /// This function deletes all undirected edges and nodes of the graph.

       /// It also frees the memory allocated to store them.

       void clear() { }

       template <typename Graph>

       struct Constraints {

 	void constraints() {}

       };

     };

     /// \brief An empty erasable undirected graph class.

     ///

     /// This class is an extension of \ref ExtendableBpUGraph. It makes it

     /// possible to erase undirected edges or nodes.

     class ErasableBpUGraph : public ExtendableBpUGraph {

     public:

       /// \brief Deletes a node.

       ///

       /// Deletes a node.

       ///

       void erase(Node) { }

       /// \brief Deletes an undirected edge.

       ///

       /// Deletes an undirected edge.

       ///

       void erase(UEdge) { }

       template <typename Graph>

       struct Constraints {

 	void constraints() {}

       };

     };