/// \param GR The type of the graph which shares its node set with

   /// \param GR The type of the graph which shares its node set with

   /// this class. Its interface must conform to the

   /// this class. Its interface must conform to the

   /// \ref concepts::Digraph "Digraph" or \ref concepts::Graph "Graph"

   /// \ref concepts::Digraph "Digraph" or \ref concepts::Graph "Graph"

   /// concept.

   /// concept.

   ///

   ///

   /// "Digraph" concept.

   /// "Digraph" concept.

   template <typename GR>

   template <typename GR>

   class ListArcSet : public ArcSetExtender<ListArcSetBase<GR> > {

   class ListArcSet : public ArcSetExtender<ListArcSetBase<GR> > {

   public:

   public:

     /// \brief Add a new arc to the digraph.

     /// \brief Add a new arc to the digraph.

     ///

     ///

     /// Add a new arc to the digraph with source node \c s

     /// Add a new arc to the digraph with source node \c s

     /// and target node \c t.

     /// and target node \c t.

     Arc addArc(const Node& s, const Node& t) {

     Arc addArc(const Node& s, const Node& t) {

       return Parent::addArc(s, t);

       return Parent::addArc(s, t);

     }

     }

     /// \brief Erase an arc from the digraph.

     /// \brief Erase an arc from the digraph.

   /// \param GR The type of the graph which shares its node set

   /// \param GR The type of the graph which shares its node set

   /// with this class. Its interface must conform to the

   /// with this class. Its interface must conform to the

   /// \ref concepts::Digraph "Digraph" or \ref concepts::Graph "Graph"

   /// \ref concepts::Digraph "Digraph" or \ref concepts::Graph "Graph"

   /// concept.

   /// concept.

   ///

   ///

   /// concept.

   /// concept.

   template <typename GR>

   template <typename GR>

   class ListEdgeSet : public EdgeSetExtender<ListEdgeSetBase<GR> > {

   class ListEdgeSet : public EdgeSetExtender<ListEdgeSetBase<GR> > {

   public:

   public:

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

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

     ///

     ///

     /// Add a new edge to the graph with node \c u

     /// Add a new edge to the graph with node \c u

     /// and node \c v endpoints.

     /// and node \c v endpoints.

     Edge addEdge(const Node& u, const Node& v) {

     Edge addEdge(const Node& u, const Node& v) {

       return Parent::addEdge(u, v);

       return Parent::addEdge(u, v);

     }

     }

     /// \brief Erase an edge from the graph.

     /// \brief Erase an edge from the graph.

   /// \warning If a node is erased from the underlying graph and this

   /// \warning If a node is erased from the underlying graph and this

   /// node is the source or target of one arc in the arc set, then

   /// node is the source or target of one arc in the arc set, then

   /// the arc set is invalidated, and it cannot be used anymore. The

   /// the arc set is invalidated, and it cannot be used anymore. The

   /// validity can be checked with the \c valid() member function.

   /// validity can be checked with the \c valid() member function.

   ///

   ///

   /// "Digraph" concept.

   /// "Digraph" concept.

   template <typename GR>

   template <typename GR>

   class SmartArcSet : public ArcSetExtender<SmartArcSetBase<GR> > {

   class SmartArcSet : public ArcSetExtender<SmartArcSetBase<GR> > {

   public:

   public:

     /// \brief Add a new arc to the digraph.

     /// \brief Add a new arc to the digraph.

     ///

     ///

     /// Add a new arc to the digraph with source node \c s

     /// Add a new arc to the digraph with source node \c s

     /// and target node \c t.

     /// and target node \c t.

     Arc addArc(const Node& s, const Node& t) {

     Arc addArc(const Node& s, const Node& t) {

       return Parent::addArc(s, t);

       return Parent::addArc(s, t);

     }

     }

     /// \brief Validity check

     /// \brief Validity check

   /// \warning If a node is erased from the underlying graph and this

   /// \warning If a node is erased from the underlying graph and this

   /// node is incident to one edge in the edge set, then the edge set

   /// node is incident to one edge in the edge set, then the edge set

   /// is invalidated, and it cannot be used anymore. The validity can

   /// is invalidated, and it cannot be used anymore. The validity can

   /// be checked with the \c valid() member function.

   /// be checked with the \c valid() member function.

   ///

   ///

   /// "Graph" concept.

   /// "Graph" concept.

   template <typename GR>

   template <typename GR>

   class SmartEdgeSet : public EdgeSetExtender<SmartEdgeSetBase<GR> > {

   class SmartEdgeSet : public EdgeSetExtender<SmartEdgeSetBase<GR> > {

   public:

   public:

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

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

     ///

     ///

     /// Add a new edge to the graph with node \c u

     /// Add a new edge to the graph with node \c u

     /// and node \c v endpoints.

     /// and node \c v endpoints.

     Edge addEdge(const Node& u, const Node& v) {

     Edge addEdge(const Node& u, const Node& v) {

       return Parent::addEdge(u, v);

       return Parent::addEdge(u, v);

     }

     }

     /// \brief Validity check

     /// \brief Validity check