namespace skeleton {

   // @defgroup empty_graph The GraphSkeleton class

   // @{

     // @defgroup empty_graph The GraphSkeleton class

     // @{

   /// An empty graph class.

     /// An empty static graph class.

   /// This class provides all the common features of a graph structure,

   /// however completely without implementations and real data structures

     /// This class provides all the common features of a graph structure,

   /// behind the interface.

     /// however completely without implementations and real data structures

   /// All graph algorithms should compile with this class, but it will not

     /// behind the interface.

   /// run properly, of course.

     /// All graph algorithms should compile with this class, but it will not

   ///

     /// run properly, of course.

   /// It can be used for checking the interface compatibility,

   /// or it can serve as a skeleton of a new graph structure.

   /// 

   /// Also, you will find here the full documentation of a certain graph

   /// feature, the documentation of a real graph imlementation

   /// like @ref ListGraph or

   /// @ref SmartGraph will just refer to this structure.

   class GraphSkeleton

   {

   public:

     /// Defalult constructor.

     GraphSkeleton() {}

     ///Copy consructor.

     ///\todo It is not clear, what we expect from a copy constructor.

     ///E.g. How to assign the nodes/edges to each other? What about maps?

     GraphSkeleton(const GraphSkeleton &G) {}

     /// The base type of the node iterators.

     /// This is the base type of each node iterators,

     /// thus each kind of node iterator will convert to this.

     class Node {

     public:

       /// @warning The default constructor sets the iterator

       /// to an undefined value.

       Node() {}   //FIXME

       /// Invalid constructor \& conversion.

       /// This constructor initializes the iterator to be invalid.

       /// \sa Invalid for more details.

       Node(Invalid) {}

       //Node(const Node &) {}

       /// Two iterators are equal if and only if they point to the

       /// same object or both are invalid.

       bool operator==(Node) const { return true; }

       /// \sa \ref operator==(Node n)

       ///

       bool operator!=(Node) const { return true; }

       bool operator<(Node) const { return true; }

     };

     /// This iterator goes through each node.

     /// This iterator goes through each node.

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

     /// of nodes in graph \c G of type \c Graph like this:

     /// \code

     ///int count=0;

     ///for(Graph::NodeIt n(G);G.valid(n);G.next(n)) count++;

     /// \endcode

     class NodeIt : public Node {

     public:

       /// @warning The default constructor sets the iterator

       /// to an undefined value.

       NodeIt() {} //FIXME

       /// Invalid constructor \& conversion.

       /// Initialize the iterator to be invalid

       /// \sa Invalid for more details.

       NodeIt(Invalid) {}

       /// Sets the iterator to the first node of \c G.

       NodeIt(const GraphSkeleton &) {}

       /// @warning The default constructor sets the iterator

       /// to an undefined value.

       NodeIt(const NodeIt &n) : Node(n) {}

     };

     /// The base type of the edge iterators.

     class Edge {

     public:

       /// @warning The default constructor sets the iterator

       /// to an undefined value.

       Edge() {}   //FIXME

       /// Initialize the iterator to be invalid

       Edge(Invalid) {}

       /// Two iterators are equal if and only if they point to the

       /// same object or both are invalid.

       bool operator==(Edge) const { return true; }

       bool operator!=(Edge) const { return true; }

       bool operator<(Edge) const { return true; }

     };

     /// This iterator goes trough the outgoing edges of a node.

     /// This iterator goes trough the \e outgoing edges of a certain node

     /// of a graph.

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

     /// of outgoing edges of a node \c n

     /// in graph \c G of type \c Graph as follows.

     /// \code

     ///int count=0;

     ///for(Graph::OutEdgeIt e(G,n);G.valid(e);G.next(e)) count++;

     /// \endcode

     class OutEdgeIt : public Edge {

     public:

       /// @warning The default constructor sets the iterator

       /// to an undefined value.

       OutEdgeIt() {}

       /// Initialize the iterator to be invalid

       OutEdgeIt(Invalid) {}

       /// This constructor sets the iterator to first outgoing edge.

       /// This constructor set the iterator to the first outgoing edge of

       /// node

       ///@param n the node

       ///@param G the graph

       OutEdgeIt(const GraphSkeleton &, Node) {}

     };

     /// This iterator goes trough the incoming edges of a node.

     /// This iterator goes trough the \e incoming edges of a certain node

     /// of a graph.

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

     /// of outgoing edges of a node \c n

     /// in graph \c G of type \c Graph as follows.

     /// \code

     ///int count=0;

     ///for(Graph::InEdgeIt e(G,n);G.valid(e);G.next(e)) count++;

     /// \endcode

     class InEdgeIt : public Edge {

     public:

       /// @warning The default constructor sets the iterator

       /// to an undefined value.

       InEdgeIt() {}

       /// Initialize the iterator to be invalid

       InEdgeIt(Invalid) {}

       InEdgeIt(const GraphSkeleton &, Node) {}    

     };

     //  class SymEdgeIt : public Edge {};

     /// This iterator goes through each edge.

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

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

     /// of edges in a graph \c G of type \c Graph as follows:

     /// \code

     ///int count=0;

     ///for(Graph::EdgeIt e(G);G.valid(e);G.next(e)) count++;

     /// \endcode

     class EdgeIt : public Edge {

     public:

       /// @warning The default constructor sets the iterator

       /// to an undefined value.

       EdgeIt() {}

       /// Initialize the iterator to be invalid

       EdgeIt(Invalid) {}

       EdgeIt(const GraphSkeleton &) {}

     };

     /// First node of the graph.

     /// \retval i the first node.

     /// \return the first node.

     NodeIt &first(NodeIt &i) const { return i;}

     /// It can be used for checking the interface compatibility,

     /// or it can serve as a skeleton of a new graph structure.

     /// The first incoming edge.

     /// 

     InEdgeIt &first(InEdgeIt &i, Node) const { return i;}

     /// Also, you will find here the full documentation of a certain graph

     /// The first outgoing edge.

     /// feature, the documentation of a real graph imlementation

     OutEdgeIt &first(OutEdgeIt &i, Node) const { return i;}

     /// like @ref ListGraph or

     //  SymEdgeIt &first(SymEdgeIt &, Node) const { return i;}

     /// @ref SmartGraph will just refer to this structure.

     /// The first edge of the Graph.

     class StaticGraphSkeleton

     EdgeIt &first(EdgeIt &i) const { return i;}

 //     Node getNext(Node) const {}

 //     InEdgeIt getNext(InEdgeIt) const {}

 //     OutEdgeIt getNext(OutEdgeIt) const {}

 //     //SymEdgeIt getNext(SymEdgeIt) const {}

 //     EdgeIt getNext(EdgeIt) const {}

     /// Go to the next node.

     NodeIt &next(NodeIt &i) const { return i;}

     /// Go to the next incoming edge.

     InEdgeIt &next(InEdgeIt &i) const { return i;}

     /// Go to the next outgoing edge.

     OutEdgeIt &next(OutEdgeIt &i) const { return i;}

     //SymEdgeIt &next(SymEdgeIt &) const {}

     /// Go to the next edge.

     EdgeIt &next(EdgeIt &i) const { return i;}

     ///Gives back the head node of an edge.

     Node head(Edge) const { return INVALID; }

     ///Gives back the tail node of an edge.

     Node tail(Edge) const { return INVALID; }

     //   Node aNode(InEdgeIt) const {}

     //   Node aNode(OutEdgeIt) const {}

     //   Node aNode(SymEdgeIt) const {}

     //   Node bNode(InEdgeIt) const {}

     //   Node bNode(OutEdgeIt) const {}

     //   Node bNode(SymEdgeIt) const {}

     /// Checks if a node iterator is valid

     ///\todo Maybe, it would be better if iterator converted to

     ///bool directly, as Jacint prefers.

     bool valid(const Node&) const { return true;}

     /// Checks if an edge iterator is valid

     ///\todo Maybe, it would be better if iterator converted to

     ///bool directly, as Jacint prefers.

     bool valid(const Edge&) const { return true;}

     ///Gives back the \e id of a node.

     ///\warning Not all graph structures provide this feature.

     ///

     int id(const Node&) const { return 0;}

     ///Gives back the \e id of an edge.

     ///\warning Not all graph structures provide this feature.

     ///

     int id(const Edge&) const { return 0;}

     //void setInvalid(Node &) const {};

     //void setInvalid(Edge &) const {};

     ///Add a new node to the graph.

     /// \return the new node.

     ///

     Node addNode() { return INVALID;}

     ///Add a new edge to the graph.

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

     ///and head node \c head.

     ///\return the new edge.

     Edge addEdge(Node, Node) { return INVALID;}

     /// Resets the graph.

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

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

     void clear() {}

     int nodeNum() const { return 0;}

     int edgeNum() const { return 0;}

     ///Read/write/reference map of the nodes to type \c T.

     ///Read/write/reference map of the nodes to type \c T.

     /// \sa MemoryMapSkeleton

     /// \todo We may need copy constructor

     /// \todo We may need conversion from other nodetype

     /// \todo We may need operator=

     /// \warning Making maps that can handle bool type (NodeMap<bool>)

     /// needs extra attention!

     template<class T> class NodeMap

       typedef T ValueType;

       /// Defalult constructor.

       typedef Node KeyType;

       StaticGraphSkeleton() {}

       ///Copy consructor.

       NodeMap(const GraphSkeleton &) {}

       NodeMap(const GraphSkeleton &, T) {}

       ///\todo It is not clear, what we expect from a copy constructor.

       ///E.g. How to assign the nodes/edges to each other? What about maps?

       template<typename TT> NodeMap(const NodeMap<TT> &) {}

       StaticGraphSkeleton(const StaticGraphSkeleton &G) {}

       /// Sets the value of a node.

       /// The base type of the node iterators.

       /// Sets the value associated with node \c i to the value \c t.

       /// This is the base type of each node iterators,

       /// thus each kind of node iterator will convert to this.

       class Node {

       public:

 	/// @warning The default constructor sets the iterator

 	/// to an undefined value.

 	Node() {}   //FIXME

 	/// Invalid constructor \& conversion.

 	/// This constructor initializes the iterator to be invalid.

 	/// \sa Invalid for more details.

 	Node(Invalid) {}

 	//Node(const Node &) {}

 	/// Two iterators are equal if and only if they point to the

 	/// same object or both are invalid.

 	bool operator==(Node) const { return true; }

 	/// \sa \ref operator==(Node n)

 	///

 	bool operator!=(Node) const { return true; }

 	bool operator<(Node) const { return true; }

       };

       /// This iterator goes through each node.

       /// This iterator goes through each node.

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

       /// of nodes in graph \c G of type \c Graph like this:

       /// \code

       ///int count=0;

       ///for(Graph::NodeIt n(G);G.valid(n);G.next(n)) count++;

       /// \endcode

       class NodeIt : public Node {

       public:

 	/// @warning The default constructor sets the iterator

 	/// to an undefined value.

 	NodeIt() {} //FIXME

 	/// Invalid constructor \& conversion.

 	/// Initialize the iterator to be invalid

 	/// \sa Invalid for more details.

 	NodeIt(Invalid) {}

 	/// Sets the iterator to the first node of \c G.

 	NodeIt(const StaticGraphSkeleton &) {}

 	/// @warning The default constructor sets the iterator

 	/// to an undefined value.

 	NodeIt(const NodeIt &n) : Node(n) {}

       };

       /// The base type of the edge iterators.

       class Edge {

       public:

 	/// @warning The default constructor sets the iterator

 	/// to an undefined value.

 	Edge() {}   //FIXME

 	/// Initialize the iterator to be invalid

 	Edge(Invalid) {}

 	/// Two iterators are equal if and only if they point to the

 	/// same object or both are invalid.

 	bool operator==(Edge) const { return true; }

 	bool operator!=(Edge) const { return true; }

 	bool operator<(Edge) const { return true; }

       };

       /// This iterator goes trough the outgoing edges of a node.

       /// This iterator goes trough the \e outgoing edges of a certain node

       /// of a graph.

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

       /// of outgoing edges of a node \c n

       /// in graph \c G of type \c Graph as follows.

       /// \code

       ///int count=0;

       ///for(Graph::OutEdgeIt e(G,n);G.valid(e);G.next(e)) count++;

       /// \endcode

       class OutEdgeIt : public Edge {

       public:

 	/// @warning The default constructor sets the iterator

 	/// to an undefined value.

 	OutEdgeIt() {}

 	/// Initialize the iterator to be invalid

 	OutEdgeIt(Invalid) {}

 	/// This constructor sets the iterator to first outgoing edge.

 	/// This constructor set the iterator to the first outgoing edge of

 	/// node

 	///@param n the node

 	///@param G the graph

 	OutEdgeIt(const StaticGraphSkeleton &, Node) {}

       };

       /// This iterator goes trough the incoming edges of a node.

       /// This iterator goes trough the \e incoming edges of a certain node

       /// of a graph.

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

       /// of outgoing edges of a node \c n

       /// in graph \c G of type \c Graph as follows.

       /// \code

       ///int count=0;

       ///for(Graph::InEdgeIt e(G,n);G.valid(e);G.next(e)) count++;

       /// \endcode

       class InEdgeIt : public Edge {

       public:

 	/// @warning The default constructor sets the iterator

 	/// to an undefined value.

 	InEdgeIt() {}

 	/// Initialize the iterator to be invalid

 	InEdgeIt(Invalid) {}

 	InEdgeIt(const StaticGraphSkeleton &, Node) {}    

       };

       //  class SymEdgeIt : public Edge {};

       /// This iterator goes through each edge.

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

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

       /// of edges in a graph \c G of type \c Graph as follows:

       /// \code

       ///int count=0;

       ///for(Graph::EdgeIt e(G);G.valid(e);G.next(e)) count++;

       /// \endcode

       class EdgeIt : public Edge {

       public:

 	/// @warning The default constructor sets the iterator

 	/// to an undefined value.

 	EdgeIt() {}

 	/// Initialize the iterator to be invalid

 	EdgeIt(Invalid) {}

 	EdgeIt(const StaticGraphSkeleton &) {}

       };

       /// First node of the graph.

       /// \retval i the first node.

       /// \return the first node.

       void set(Node, T) {}

       NodeIt &first(NodeIt &i) const { return i;}

       // Gets the value of a node.

       //T get(Node i) const {return *(T*)0;}  //FIXME: Is it necessary?

       /// The first incoming edge.

       T &operator[](Node) {return *(T*)0;}

       InEdgeIt &first(InEdgeIt &i, Node) const { return i;}

       const T &operator[](Node) const {return *(T*)0;}

       /// The first outgoing edge.

       OutEdgeIt &first(OutEdgeIt &i, Node) const { return i;}

       /// Updates the map if the graph has been changed

       //  SymEdgeIt &first(SymEdgeIt &, Node) const { return i;}

       /// The first edge of the Graph.

       /// \todo Do we need this?

       EdgeIt &first(EdgeIt &i) const { return i;}

       //     Node getNext(Node) const {}

       //     InEdgeIt getNext(InEdgeIt) const {}

       //     OutEdgeIt getNext(OutEdgeIt) const {}

       //     //SymEdgeIt getNext(SymEdgeIt) const {}

       //     EdgeIt getNext(EdgeIt) const {}

       /// Go to the next node.

       NodeIt &next(NodeIt &i) const { return i;}

       /// Go to the next incoming edge.

       InEdgeIt &next(InEdgeIt &i) const { return i;}

       /// Go to the next outgoing edge.

       OutEdgeIt &next(OutEdgeIt &i) const { return i;}

       //SymEdgeIt &next(SymEdgeIt &) const {}

       /// Go to the next edge.

       EdgeIt &next(EdgeIt &i) const { return i;}

       ///Gives back the head node of an edge.

       Node head(Edge) const { return INVALID; }

       ///Gives back the tail node of an edge.

       Node tail(Edge) const { return INVALID; }

       //   Node aNode(InEdgeIt) const {}

       //   Node aNode(OutEdgeIt) const {}

       //   Node aNode(SymEdgeIt) const {}

       //   Node bNode(InEdgeIt) const {}

       //   Node bNode(OutEdgeIt) const {}

       //   Node bNode(SymEdgeIt) const {}

       /// Checks if a node iterator is valid

       ///\todo Maybe, it would be better if iterator converted to

       ///bool directly, as Jacint prefers.

       bool valid(const Node&) const { return true;}

       /// Checks if an edge iterator is valid

       ///\todo Maybe, it would be better if iterator converted to

       ///bool directly, as Jacint prefers.

       bool valid(const Edge&) const { return true;}

       ///Gives back the \e id of a node.

       ///\warning Not all graph structures provide this feature.

       void update() {}

       int id(const Node&) const { return 0;}

       void update(T a) {}   //FIXME: Is it necessary

       ///Gives back the \e id of an edge.

       ///\warning Not all graph structures provide this feature.

       ///

       int id(const Edge&) const { return 0;}

       /// Resets the graph.

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

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

       void clear() {}

       int nodeNum() const { return 0;}

       int edgeNum() const { return 0;}

       ///Reference map of the nodes to type \c T.

       ///Reference map of the nodes to type \c T.

       /// \sa ReferenceSkeleton

       /// \warning Making maps that can handle bool type (NodeMap<bool>)

       /// needs extra attention!

       template<class T> class NodeMap

 	: public ReferenceMap< Node, T >

       {

       public:

 	class ReferenceMap<Node,T>;

 	NodeMap(const StaticGraphSkeleton &) {}

 	NodeMap(const StaticGraphSkeleton &, T) {}

 	///Copy constructor

 	template<typename TT> NodeMap(const NodeMap<TT> &) {}

 	///Assignment operator

 	template<typename TT> NodeMap &operator=(const NodeMap<TT> &)

 	{return *this;}

       };

       ///Reference map of the edges to type \c T.

       ///Reference map of the edges to type \c T.

       /// \sa ReferenceSkeleton

       /// \warning Making maps that can handle bool type (EdgeMap<bool>)

       /// needs extra attention!

       template<class T> class EdgeMap

 	: public ReferenceMap<Edge,T>

       {

       public:

 	typedef T ValueType;

 	typedef Edge KeyType;

 	EdgeMap(const StaticGraphSkeleton &) {}

 	EdgeMap(const StaticGraphSkeleton &, T ) {}

 	///Copy constructor

 	template<typename TT> EdgeMap(const EdgeMap<TT> &) {}

 	///Assignment operator

 	template<typename TT> EdgeMap &operator=(const EdgeMap<TT> &)

 	{return *this;}

       };

     ///Read/write/reference map of the edges to type \c T.

     ///Read/write/reference map of the edges to type \c T.

     /// An empty graph class.

     ///It behaves exactly in the same way as \ref NodeMap.

     /// \sa NodeMap

     /// This class provides everything that \c StaticGraphSkeleton

     /// \sa MemoryMapSkeleton

     /// with additional functionality which enables to build a

     /// \todo We may need copy constructor

     /// graph from scratch.

     /// \todo We may need conversion from other edgetype

     class GraphSkeleton : public StaticGraphSkeleton

     /// \todo We may need operator=

     template<class T> class EdgeMap

       typedef T ValueType;

       /// Defalult constructor.

       typedef Edge KeyType;

       GraphSkeleton() {}

       ///Copy consructor.

       EdgeMap(const GraphSkeleton &) {}

       EdgeMap(const GraphSkeleton &, T ) {}

       ///\todo It is not clear, what we expect from a copy constructor.

       ///E.g. How to assign the nodes/edges to each other? What about maps?

       ///\todo It can copy between different types.

       GraphSkeleton(const GraphSkeleton &G) {}

       ///Add a new node to the graph.

       /// \return the new node.

       template<typename TT> EdgeMap(const EdgeMap<TT> &) {}

       Node addNode() { return INVALID;}

       ///Add a new edge to the graph.

       void set(Edge, T) {}

       //T get(Edge) const {return *(T*)0;}

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

       T &operator[](Edge) {return *(T*)0;}

       ///and head node \c head.

       const T &operator[](Edge) const {return *(T*)0;}

       ///\return the new edge.

       Edge addEdge(Node, Node) { return INVALID;}

       void update() {}

       void update(T a) {}   //FIXME: Is it necessary

       /// Resets the graph.

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

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

       /// \todo It might belong to \c EraseableGraphSkeleton.

       void clear() {}

   };

     /// An empty eraseable graph class.

   /// An empty eraseable graph class.

     /// This class is an extension of \c GraphSkeleton. It also makes it

   /// This class provides all the common features of an \e eraseable graph

     /// possible to erase edges or nodes.

   /// structure,

     class EraseableGraphSkeleton : public GraphSkeleton

   /// however completely without implementations and real data structures

     {

   /// behind the interface.

     public:

   /// All graph algorithms should compile with this class, but it will not

       /// Deletes a node.

   /// run properly, of course.

       void erase(Node n) {}

   ///

       /// Deletes an edge.

   /// \todo This blabla could be replaced by a sepatate description about

       void erase(Edge e) {}

   /// Skeletons.

   ///

       /// Defalult constructor.

   /// It can be used for checking the interface compatibility,

       EraseableGraphSkeleton() {}

   /// or it can serve as a skeleton of a new graph structure.

       ///Copy consructor.

   /// 

       EraseableGraphSkeleton(const GraphSkeleton &G) {}

   /// Also, you will find here the full documentation of a certain graph

     };

   /// feature, the documentation of a real graph imlementation

   /// like @ref ListGraph or

     // @}

   /// @ref SmartGraph will just refer to this structure.

   } //namespace skeleton

   class EraseableGraphSkeleton : public GraphSkeleton

   {

   public:

     /// Deletes a node.

     void erase(Node n) {}

     /// Deletes an edge.

     void erase(Edge e) {}

     /// Defalult constructor.

     EraseableGraphSkeleton() {}

     ///Copy consructor.

     EraseableGraphSkeleton(const GraphSkeleton &G) {}

   };

   // @}