/// The directed arc type.

       /// The directed arc type.

       /// The directed arc type. It can be converted to the

       /// The directed arc type. It can be converted to the

       /// edge or it should be inherited from the undirected

       /// edge or it should be inherited from the undirected

       public:

       public:

         /// Default constructor

         /// Default constructor

         /// @warning The default constructor sets the iterator

         /// @warning The default constructor sets the iterator

         /// to an undefined value.

         /// to an undefined value.

         Arc() { }

         Arc() { }

         /// Copy constructor.

         /// Copy constructor.

         /// Copy constructor.

         /// Copy constructor.

         ///

         ///

         /// Initialize the iterator to be invalid.

         /// Initialize the iterator to be invalid.

         /// Initialize the iterator to be invalid.

         /// Initialize the iterator to be invalid.

         ///

         ///

         Arc(Invalid) { }

         Arc(Invalid) { }

         /// \note This operator only have to define some strict ordering of

         /// \note This operator only have to define some strict ordering of

         /// the items; this order has nothing to do with the iteration

         /// the items; this order has nothing to do with the iteration

         /// ordering of the items.

         /// ordering of the items.

         bool operator<(Arc) const { return false; }

         bool operator<(Arc) const { return false; }

       };

       };

       /// This iterator goes through each directed arc.

       /// This iterator goes through each directed arc.

       /// This iterator goes through each arc of a graph.

       /// This iterator goes through each arc of a graph.

       /// Its usage is quite simple, for example you can count the number

       /// Its usage is quite simple, for example you can count the number