template<char Ch>

     template<char _selector>

       /// Assign operator for nodes.

       // We explicitely list these:

       GraphItem(GraphItem const&) {}

       /// The nodes are assignable. 

       ///

       /// Equality operator.

       /// Two iterators are equal if and only if they represents the

       /// same node in the graph or both are invalid.

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

       //      bool operator<(GraphItem) const { return false; }

     };

       template<typename _GraphItem>

     template<typename GI>

       struct Constraints {

     struct GraphItemConcept {

 	void constraints() {

       void constraints() {

 	  _GraphItem i1;

 	GI i1;

 	  _GraphItem i2 = i1;

 	GI i2 = i1;

 	  _GraphItem i3 = INVALID;

 	GI i3 = INVALID;

 	  i1 = i2 = i3;

 	i1 = i2 = i3;

 	  bool b;

 	bool b;

 	  //	  b = (ia == ib) && (ia != ib) && (ia < ib);

 	b = (ia == ib) && (ia != ib) && (ia < ib);

 	  b = (ia == ib) && (ia != ib);

 	b = (ia == INVALID) && (ib != INVALID);

 	  b = (ia == INVALID) && (ib != INVALID);

       }

 	}

       const GI &ia;

 	const _GraphItem &ia;

       const GI &ib;

 	const _GraphItem &ib;

     };

       };

     };

     template<typename Iter, typename Graph, typename BaseItem>

     struct GraphIteratorConcept {

       void constraints() {

 	function_requires< GraphItemConcept<Iter> >();

 	Iter it1(g);

 	/// \todo Do we need NodeIt(Node) kind of constructor?

 	//	Iter it2(bj);

 	Iter it2;

 	it2 = ++it1;

 	++it2 = it1;

 	++(++it1);

 	/// \bug This should be: is_base_and_derived<BaseItem, Iter>

 	BaseItem bi = it1;

 	bi = it2;

       }

       BaseItem bj;

       Graph g;

     };

     template<typename Iter, typename Graph>

     struct GraphIncIteratorConcept {

       typedef typename Graph::Node Node;

       typedef typename Graph::Edge Edge;

       void constraints() {

 	function_requires< GraphItemConcept<Iter> >();

 	Iter it1(graph, node);

 	/// \todo Do we need OutEdgeIt(Edge) kind of constructor?

 	//	Iter it2(edge);

 	Iter it2;

 	it2 = ++it1;

 	++it2 = it1;

 	++(++it1);

 	Edge e = it1;

 	e = it2;

       }

       Edge edge;

       Node node;

       Graph graph;

     };

       class Node {

       typedef GraphItem<'n'> Node;

       public:

 	/// Default constructor.

 	/// @warning The default constructor sets the iterator

 	/// to an undefined value.

 	Node() {}

 	/// Copy constructor.

 	/// Copy constructor.

 	///

 	Node(const Node&) {}

 	/// Invalid constructor \& conversion.

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

 	/// \sa Invalid for more details.

 	Node(Invalid) {}

 	/// Assign operator for nodes.

 	/// The nodes are assignable. 

 	///

 	Node& operator=(const Node&) { return *this;}

 	/// Equality operator.

 	/// Two iterators are equal if and only if they represents the

 	/// same node in the graph or both are invalid.

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

 	/// Inequality operator.

 	/// \sa operator==(const Node& n)

 	///

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

  	/// Comparison operator.

 	/// This is a strict ordering between the nodes.

 	///

 	/// This ordering can be different from the iterating order of nodes.

 	/// \todo Possibly we don't need it.

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

       };

       class Edge {

       typedef GraphItem<'e'> Edge;

       public:

 	/// Default constructor.

 	/// @warning The default constructor sets the iterator

 	/// to an undefined value.

 	Edge() {}

 	/// Copy constructor.

 	/// Copy constructor.

 	///

 	Edge(const Edge&) {}

 	/// Invalid constructor \& conversion.

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

 	/// \sa Invalid for more details.

 	Edge(Invalid) {}

 	/// Assign operator for edges.

 	/// The edges are assignable. 

 	///

 	Edge& operator=(const Edge&) { return *this;}

 	/// Equality operator.

 	/// Two iterators are equal if and only if they represents the

 	/// same edge in the graph or both are invalid.

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

 	/// Inequality operator.

 	/// \sa operator==(const Edge& n)

 	///

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

  	/// Comparison operator.

 	/// This is a strict ordering between the edges.

 	///

 	/// This ordering can be different from the iterating order of edges.

 	/// \todo Possibly we don't need it.

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

       };

     };

       template <typename _Graph>

     /// Concept check structure for BaseGraph.

       struct Constraints {

 	typedef typename _Graph::Node Node;

     /// Concept check structure for BaseGraph.

 	typedef typename _Graph::Edge Edge;

     ///

 	void constraints() {

     template <typename Graph>

 	  checkConcept<GraphItem<'n'>, Node>();

     struct BaseGraphComponentConcept {

 	  checkConcept<GraphItem<'e'>, Edge>();

       typedef typename Graph::Node Node;

 	  {

       typedef typename Graph::Edge Edge;

 	    Node n;

 	    Edge e;

       void constraints() {

 	    n = graph.source(e);

 	function_requires< GraphItemConcept<Node> >();

 	    n = graph.target(e);

 	function_requires< GraphItemConcept<Edge> >();

 	  }      

 	{

 	}

 	  Node n;

 	  Edge e;

 	const _Graph& graph;

 	  n = graph.source(e);

       };

 	  n = graph.target(e);

 	}      

       }

       const Graph& graph;

     };

       template <typename _Graph>

     /// Concept check structure for IterableBaseGraph.

       struct Constraints {

     /// Concept check structure for IterableBaseGraph.

 	void constraints() {

     ///

 	  checkConcept< BaseGraphComponent, _Graph >();

     template <typename Graph>

 	  typename _Graph::Node node;      

     struct BaseIterableGraphComponentConcept {

 	  typename _Graph::Edge edge;

 	  {

       void constraints() {

 	    graph.first(node);

 	function_requires< BaseGraphComponentConcept<Graph> >();

 	    graph.next(node);

 	typename Graph::Node node;      

 	  }

 	typename Graph::Edge edge;

 	  {

 	{

 	    graph.first(edge);

 	  graph.first(node);

 	    graph.next(edge);

 	  graph.next(node);

 	  }

 	}

 	  {

 	{

 	    graph.firstIn(edge, node);

 	  graph.first(edge);

 	    graph.nextIn(edge);

 	  graph.next(edge);

 	  }

 	}

 	  {

 	{

 	    graph.firstOut(edge, node);

 	  graph.firstIn(edge, node);

 	    graph.nextOut(edge);

 	  graph.nextIn(edge);

 	  }

 	}

 	}

 	{

 	  graph.firstOut(edge, node);

 	const _Graph& graph;

 	  graph.nextOut(edge);

       };

 	}

       }

       const Graph& graph;

     };

       template <typename _Graph>

       struct Constraints {

     /// Concept check structure for IdableBaseGraph.

 	void constraints() {

     /// Concept check structure for IdableBaseGraph.

 	  checkConcept< BaseGraphComponent, _Graph >();

     ///

 	  typename _Graph::Node node;

     template <typename Graph>

 	  int nid = graph.id(node);

     struct IDableGraphComponentConcept {

 	  nid = graph.id(node);

 	  typename _Graph::Edge edge;

       void constraints() {

 	  int eid = graph.id(edge);

 	function_requires< BaseGraphComponentConcept<Graph> >();

 	  eid = graph.id(edge);

 	typename Graph::Node node;

 	}

 	int nid = graph.id(node);

 	nid = graph.id(node);

 	const _Graph& graph;

 	typename Graph::Edge edge;

       };

 	int eid = graph.id(edge);

 	eid = graph.id(edge);

       }

       const Graph& graph;

     };

       template <typename _Graph>

     /// Concept check structure for MaxIdableBaseGraph.

       struct Constraints {

     /// Concept check structure for MaxIdableBaseGraph.

 	void constraints() {

     ///

 	  checkConcept<BaseGraphComponent, _Graph>();

     template <typename Graph>

 	  int nid = graph.maxId(typename _Graph::Node());

     struct MaxIDableGraphComponentConcept {

 	  ignore_unused_variable_warning(nid);

 	  int eid = graph.maxId(typename _Graph::Edge());

       void constraints() {

 	  ignore_unused_variable_warning(eid);

 	function_requires< BaseGraphComponentConcept<Graph> >();

 	}

 	int nid = graph.maxId(typename Graph::Node());

 	ignore_unused_variable_warning(nid);

 	const _Graph& graph;

 	int eid = graph.maxId(typename Graph::Edge());

       };

 	ignore_unused_variable_warning(eid);

       }

       const Graph& graph;

     };

       template <typename _Graph>

       struct Constraints {

     /// Concept check structure for ExtendableBaseGraph.

 	void constraints() {

 	  checkConcept<BaseGraphComponent, _Graph >();

     /// Concept check structure for ExtendableBaseGraph.

 	  typename _Graph::Node node_a, node_b;

     ///

 	  node_a = graph.addNode();

     template <typename Graph>

 	  typename _Graph::Edge edge;

     struct BaseExtendableGraphComponentConcept {

 	  edge = graph.addEdge(node_a, node_b);

       void constraints() {

 	}

 	function_requires< BaseGraphComponentConcept<Graph> >();

 	typename Graph::Node node_a, node_b;

 	_Graph& graph;

 	node_a = graph.addNode();

       };

 	typename Graph::Edge edge;

 	edge = graph.addEdge(node_a, node_b);

       }

       Graph& graph;

     };

       template <typename _Graph>

       struct Constraints {

     /// Concept check structure for ErasableBaseGraph.

 	void constraints() {

 	  checkConcept<BaseGraphComponent, _Graph>();

     /// Concept check structure for ErasableBaseGraph.

 	  typename _Graph::Node node;

     ///

 	  graph.erase(node);

     template <typename Graph>

 	  typename _Graph::Edge edge;

     struct BaseErasableGraphComponentConcept {

 	  graph.erase(edge);

       void constraints() {

 	}

 	function_requires< BaseGraphComponentConcept<Graph> >();

 	typename Graph::Node node;

 	_Graph& graph;

 	graph.erase(node);

       };

 	typename Graph::Edge edge;

 	graph.erase(edge);

       }

       Graph& graph;

     };

       template <typename _Graph>

     /// Concept check function for ErasableBaseGraph.

       struct Constraints {

 	void constraints() {

     /// Concept check function for ErasableBaseGraph.

 	  checkConcept< BaseGraphComponent, _Graph>();

 	  graph.clear();

 	}

 	_Graph& graph;

       };

     };

     /// Skeleton class for graph NodeIt and EdgeIt

     /// Skeleton class for graph NodeIt and EdgeIt.

     template <typename Graph>

     template <typename _Graph, typename _Item>

     struct BaseClearableGraphComponentConcept {

     class GraphIterator : public _Item {

       void constraints() {

     public:

 	function_requires< BaseGraphComponentConcept<Graph> >();

       /// \todo Don't we need the Item type as typedef?

 	graph.clear();

       }

       /// Default constructor.

       Graph& graph;

       /// @warning The default constructor sets the iterator

     };

       /// to an undefined value.

       GraphIterator() {}

       /// Copy constructor.

     class IterableGraphComponent :

       virtual public BaseIterableGraphComponent {

       /// Copy constructor.

       ///

       GraphIterator(GraphIterator const&) {}

       /// Sets the iterator to the first item.

       /// Sets the iterator to the first item of \c the graph.

       ///

       explicit GraphIterator(const _Graph&) {}

       /// Invalid constructor \& conversion.

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

       /// \sa Invalid for more details.

       GraphIterator(Invalid) {}

       /// Assign operator for items.

       /// The items are assignable. 

       ///

       GraphIterator& operator=(GraphIterator const&) { return *this; }      

       /// Next item.

       /// Assign the iterator to the next item.

       ///

       GraphIterator& operator++() { return *this; }

       //	Node operator*() const { return INVALID; }

       /// Equality operator

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

       /// same object or both are invalid.

       bool operator==(const GraphIterator&) const { return true;}

       /// Inequality operator

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

       ///

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

       template<typename _GraphIterator>

       struct Constraints {

 	void constraints() {

 	  //	  checkConcept< Item, _GraphIterator >();

 	  _GraphIterator it1(g);

 	  /// \todo Do we need NodeIt(Node) kind of constructor?

 	  //	_GraphIterator it2(bj);

 	  _GraphIterator it2;

 	  it2 = ++it1;

 	  ++it2 = it1;

 	  ++(++it1);

 	  /// \bug This should be: is_base_and_derived<BaseItem, _GraphIterator>

 	  _Item bi = it1;

 	  bi = it2;

 	}

 	_Graph& g;

       };

     };

     /// Skeleton class for graph InEdgeIt and OutEdgeIt

     /// \note Because InEdgeIt and OutEdgeIt may not inherit from the same

     /// base class, the _selector is a additional template parameter. For 

     /// InEdgeIt you should instantiate it with character 'i' and for 

     /// OutEdgeIt with 'o'.

     /// \todo check the name of the concept GraphIncEdgeIterator

     template <typename _Graph, char _selector>

     class GraphIncEdgeIterator : public _Graph::Edge {

     public:

       /// Default constructor.

       /// @warning The default constructor sets the iterator

       /// to an undefined value.

       GraphIncEdgeIterator() {}

       /// Copy constructor.

       /// Copy constructor.

       ///

       GraphIncEdgeIterator(GraphIncEdgeIterator const&) {}

       /// Sets the iterator to the first edge incoming into or outgoing from the node.

       /// Sets the iterator to the first edge incoming into or outgoing from the node.

       ///

       explicit GraphIncEdgeIterator(const _Graph&, const typename _Graph::Node&) {}

       /// Invalid constructor \& conversion.

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

       /// \sa Invalid for more details.

       GraphIncEdgeIterator(Invalid) {}

       /// Assign operator for nodes.

       /// The nodes are assignable. 

       ///

       GraphIncEdgeIterator& operator=(GraphIncEdgeIterator const&) { return *this; }      

       /// Next edge.

       /// Assign the iterator to the next node.

       ///

       GraphIncEdgeIterator& operator++() { return *this; }

       //	Node operator*() const { return INVALID; }

       /// Equality operator

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

       /// same object or both are invalid.

       bool operator==(const GraphIncEdgeIterator&) const { return true;}

       /// Inequality operator

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

       ///

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

       template <typename _GraphIncEdgeIterator>

       struct Constraints {

 	typedef typename _Graph::Node Node;

 	typedef typename _Graph::Edge Edge;

 	void constraints() {

 	  checkConcept<GraphItem<'e'>, _GraphIncEdgeIterator>();

 	  _GraphIncEdgeIterator it1(graph, node);

 	  /// \todo Do we need OutEdgeIt(Edge) kind of constructor?

 	  //	_GraphIncEdgeIterator it2(edge);

 	  _GraphIncEdgeIterator it2;

 	  it2 = ++it1;

 	  ++it2 = it1;

 	  ++(++it1);

 	  Edge e = it1;

 	  e = it2;

 	}

 	Edge& edge;

 	Node& node;

 	_Graph& graph;

       };

     };

     /// An empty iterable base graph class.

     /// This class provides beside the core graph features

     /// iterator based iterable interface for the graph structure.

     /// This concept is part of the StaticGraphConcept.

     class IterableGraphComponent : virtual public BaseGraphComponent {

       class NodeIt : public Node {

       /// This iterator goes through each node.

       public:

 	NodeIt() {}

       /// This iterator goes through each node.

 	NodeIt(Invalid) {}

       ///

 	// explicit NodeIt(Node) {}

       typedef GraphIterator<Graph, Node> NodeIt;

 	explicit NodeIt(const Graph&) {}

       /// This iterator goes through each node.

 	NodeIt& operator++() { return *this; }

       /// This iterator goes through each node.

 	//	Node operator*() const { return INVALID; }

       ///

       typedef GraphIterator<Graph, Edge> EdgeIt;

 	bool operator==(const NodeIt&) const { return true;}

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

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

       };

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

       /// of a graph.

       class EdgeIt : public Edge {

       typedef GraphIncEdgeIterator<Graph, 'i'> InEdgeIt;

       public:

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

 	EdgeIt() {}

 	EdgeIt(Invalid) {}

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

 	// explicit EdgeIt(Edge) {}

       /// of a graph.

 	explicit EdgeIt(const Graph&) {}

       typedef GraphIncEdgeIterator<Graph, 'o'> OutEdgeIt;

 	EdgeIt& operator++() { return *this; }

 	//	Edge operator*() const { return INVALID; }

 	bool operator==(const EdgeIt&) const { return true;}

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

       };

       class InEdgeIt : public Edge {

       public:

 	InEdgeIt() {}

 	InEdgeIt(Invalid) {}

 	// explicit InEdgeIt(Edge) {}

 	explicit InEdgeIt(const Graph&, const Node&) {}

 	InEdgeIt& operator++() { return *this; }

 	//	Edge operator*() const { return INVALID; }

 	bool operator==(const InEdgeIt&) const { return true;}

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

       };

       class OutEdgeIt : public Edge {

       public:

 	OutEdgeIt() {}

 	OutEdgeIt(Invalid) {}

 	// explicit OutEdgeIt(Edge) {}

 	explicit OutEdgeIt(const Graph&, const Node&) {}

 	OutEdgeIt& operator++() { return *this; }

 	//	Edge operator*() const { return INVALID; }

 	bool operator==(const OutEdgeIt&) const { return true;}

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

       };

     template <typename Graph> 

     template <typename _Graph> 

     struct IterableGraphComponentConcept {

     struct Constraints {

   	function_requires< BaseIterableGraphComponentConcept<Graph> >();

   	checkConcept< BaseGraphComponent, _Graph>();

 	typedef typename Graph::Node Node;

 	checkConcept<GraphIterator<_Graph, typename _Graph::Edge>, typename _Graph::EdgeIt >();

 	typedef typename Graph::NodeIt NodeIt;

 	checkConcept<GraphIterator<_Graph, typename _Graph::Node>, typename _Graph::NodeIt >();

 	typedef typename Graph::Edge Edge;

 	checkConcept<GraphIncEdgeIterator<_Graph, 'i'>, typename _Graph::InEdgeIt >();

 	typedef typename Graph::EdgeIt EdgeIt;

 	checkConcept<GraphIncEdgeIterator<_Graph, 'o'>, typename _Graph::OutEdgeIt >();

 	typedef typename Graph::InEdgeIt InEdgeIt;

       }

 	typedef typename Graph::OutEdgeIt OutEdgeIt;

     };

     template <typename Graph, typename Item, typename _Value>

     class GraphMap : public ReadWriteMap<Item, _Value> {

     protected:      

       GraphMap() {}

     public:

       explicit GraphMap(const Graph&) {}

       GraphMap(const Graph&, const _Value&) {}

       GraphMap(const GraphMap&) {}

       GraphMap& operator=(const GraphMap&) { return *this;}

       template<typename _Map>

       struct Constraints {

 	void constraints() {

 	  checkConcept<ReadWriteMap<Item, _Value>, _Map >();

 	  // Construction with a graph parameter

 	  _Map a(g);

 	  // Constructor with a graph and a default value parameter

 	  _Map a2(g,t);

 	  // Copy constructor. Do we need it?

 	  _Map b=c;

 	  // Copy operator. Do we need it?

 	  a=b;

 	  ignore_unused_variable_warning(a2);

 	}

 	const _Map &c;

 	const Graph &g;

 	const typename GraphMap::Value &t;

       };

     };

     /// An empty mappable base graph class.

 	function_requires< GraphIteratorConcept<NodeIt, Graph, Node> >();

     /// This class provides beside the core graph features

 	function_requires< GraphIteratorConcept<EdgeIt, Graph, Edge> >();

     /// map interface for the graph structure.

 	function_requires< GraphIncIteratorConcept<OutEdgeIt, Graph> >();

     /// This concept is part of the StaticGraphConcept.

 	function_requires< GraphIncIteratorConcept<InEdgeIt, Graph> >();

       }

     };

       class NodeMap : public ReferenceMap<Node, _Value> {

       class NodeMap : public GraphMap<Graph, Node, _Value> {

       private:

 	NodeMap();

 	NodeMap(const Graph&) {}

 	// \todo call the right parent class constructor

 	explicit NodeMap(const Graph&) {}

       };

       };

       /// ReadWrite map of the edges.

       /// ReadWrite map of the edges.

       ///

       class EdgeMap : public ReferenceMap<Edge, _Value> {

       class EdgeMap : public GraphMap<Graph, Edge, _Value> {

       private:

 	EdgeMap();

 	EdgeMap(const Graph&) {}

 	// \todo call the right parent class constructor

 	explicit EdgeMap(const Graph&) {}

       };

       };

     };

       template <typename _Graph>

       struct Constraints {

     template <typename Graph>

     struct MappableGraphComponentConcept {

 	struct Type {

 	  int value;

       struct Type {

 	  Type() : value(0) {}

 	int value;

 	  Type(int _v) : value(_v) {}

 	Type() : value(0) {}

 	};

 	Type(int _v) : value(_v) {}

       };

 	void constraints() {

 	  checkConcept<BaseGraphComponent, _Graph>();

       void constraints() {

 	  { // int map test

 	function_requires< BaseGraphComponentConcept<Graph> >();

 	    typedef typename _Graph::template NodeMap<int> IntNodeMap;

 	{ // int map test

 	    checkConcept<GraphMap<_Graph, typename _Graph::Node, int>, IntNodeMap >();

 	  typedef typename Graph::template NodeMap<int> IntNodeMap;

 	  } { // bool map test

 	  function_requires<GraphMapConcept<IntNodeMap,Graph> >();

 	    typedef typename _Graph::template NodeMap<bool> BoolNodeMap;

 	} { // bool map test

 	    checkConcept<GraphMap<_Graph, typename _Graph::Node, bool>, BoolNodeMap >();

 	  typedef typename Graph::template NodeMap<bool> BoolNodeMap;

 	  } { // Type map test

 	  function_requires<GraphMapConcept<BoolNodeMap,Graph> >();

 	    typedef typename _Graph::template NodeMap<Type> TypeNodeMap;

 	} { // Type map test

 	    checkConcept<GraphMap<_Graph, typename _Graph::Node, Type>, TypeNodeMap >();

 	  typedef typename Graph::template NodeMap<Type> TypeNodeMap;

 	  } 

 	  function_requires<GraphMapConcept<TypeNodeMap,Graph> >();

 	} 

 	  { // int map test

 	    typedef typename _Graph::template EdgeMap<int> IntEdgeMap;

 	{ // int map test

 	    checkConcept<GraphMap<_Graph, typename _Graph::Edge, int>, IntEdgeMap >();

 	  typedef typename Graph::template EdgeMap<int> IntEdgeMap;

 	  } { // bool map test

 	  function_requires<GraphMapConcept<IntEdgeMap,Graph> >();

 	    typedef typename _Graph::template EdgeMap<bool> BoolEdgeMap;

 	} { // bool map test

 	    checkConcept<GraphMap<_Graph, typename _Graph::Edge, bool>, BoolEdgeMap >();

 	  typedef typename Graph::template EdgeMap<bool> BoolEdgeMap;

 	  } { // Type map test

 	  function_requires<GraphMapConcept<BoolEdgeMap,Graph> >();

 	    typedef typename _Graph::template EdgeMap<Type> TypeEdgeMap;

 	} { // Type map test

 	    checkConcept<GraphMap<_Graph, typename _Graph::Edge, Type>, TypeEdgeMap >();

 	  typedef typename Graph::template EdgeMap<Type> TypeEdgeMap;

 	  } 

 	  function_requires<GraphMapConcept<TypeEdgeMap,Graph> >();

 	}

 	} 

       }

 	_Graph& graph;

       };

       Graph& graph;

     };

       template <typename _Graph>

       struct Constraints {

     template <typename Graph>

 	void constraints() {

     struct ExtendableGraphComponentConcept {

 	  checkConcept<BaseGraphComponent, _Graph >();

       void constraints() {

 	  typename _Graph::Node node_a, node_b;

 	function_requires< BaseGraphComponentConcept<Graph> >();

 	  node_a = graph.addNode();

 	typename Graph::Node node_a, node_b;

 	  typename _Graph::Edge edge;

 	node_a = graph.addNode();

 	  edge = graph.addEdge(node_a, node_b);      

 	typename Graph::Edge edge;

 	}

 	edge = graph.addEdge(node_a, node_b);      

 	_Graph& graph;

       }

       };

       Graph& graph;

     };

     };

     };

       template <typename _Graph>

       struct Constraints {

     template <typename Graph>

 	void constraints() {

     struct ErasableGraphComponentConcept {

 	  checkConcept<BaseGraphComponent, _Graph >();

       void constraints() {

 	  typename _Graph::Node node;

 	function_requires< BaseGraphComponentConcept<Graph> >();

 	  graph.erase(node);

 	typename Graph::Node node;

 	  typename _Graph::Edge edge;

 	graph.erase(node);

 	  graph.erase(edge);      

 	typename Graph::Edge edge;

 	}

 	graph.erase(edge);      

       }

 	_Graph& graph;

       };

       Graph& graph;

     };

       template <typename _Graph>

     template <typename Graph>

       struct ClearableGraphComponentConcept {

     struct ClearableGraphComponentConcept {

 	void constraints() {

       void constraints() {

 	  checkConcept< BaseGraphComponent, _Graph >();

 	function_requires< BaseGraphComponentConcept<Graph> >();

 	  graph.clear();

 	graph.clear();

 	}

       }

 	_Graph& graph;

       Graph& graph;

       };