/// \todo Implement!

     DirPath(const DirPath &P, const NodeIt &a, const NodeIt &b) {

     DirPath(const DirPath &P, const NodeIt &a, const NodeIt &b);

       if( DM::range_check && (!a.valid() || !b.valid) ) {

 	// FIXME: this check should be more elaborate...

 	fault("DirPath, subpath ctor: invalid bounding nodes");

       }

       gr = P.gr;

       edges.insert(edges.end(), P.edges.begin()+a.idx, P.edges.begin()+b.idx);

     }

     /// \todo Implement!

     DirPath(const DirPath &P, const EdgeIt &a, const EdgeIt &b) {

     DirPath(const DirPath &P, const EdgeIt &a, const EdgeIt &b);

       if( DM::range_check && (!a.valid() || !b.valid) ) {

 	// FIXME: this check should be more elaborate...

 	fault("DirPath, subpath ctor: invalid bounding nodes");

       }

       gr = P.gr;

       edges.insert(edges.end(), P.edges.begin()+a.idx, P.edges.begin()+b.idx);

     }

     /// \brief Initializes node or edge iterator to point to the node or edge

     /// \brief Initializes node iterator to point to the node of a given index.

     /// of a given index.

     NodeIt& nth(NodeIt &i, int n) const {

     template<typename It>

     It& nth(It &i, int n) const {

       // FIXME: this test should be different for NodeIt and EdgeIt:

       return i=It(*this, n);

       return i=NodeIt(*this, n);

     }

     /// \brief Initializes edge iterator to point to the edge of a given index.

     EdgeIt& nth(EdgeIt &i, int n) const {

       if( DM::range_check && (n<0 || n>=int(length())) )

 	fault("DirPath::nth: index out of range");

       return i=EdgeIt(*this, n);

     bool valid(const It &i) const { return i.valid(); }

     static

     bool valid(const It &i) { return i.valid(); }

     It& next(It &e) const {

     static

     It& next(It &e) {

       operator const GraphEdge& () const {

       operator const GraphNode& () const {

      * arbitrary order the you can commit these changes to the graph.

      * arbitrary order then you should commit these changes to the graph.

       ///Sets the first node of the path.

       /// Sets the starting node of the path.

       ///Sets the first node of the path. If the path is empty, this

       /// Sets the starting node of the path. Edge added to the path

       ///function or setTo() have to be called before any call to \ref

       /// afterwards have to be incident to this node.

       ///pushFront() or \ref pushBack()

       /// It should be called iff the path is empty and before any call to

       void setFrom(const GraphNode &) {}

       /// \ref pushFront() or \ref pushBack()

       void setStart(const GraphNode &) {}

       ///Sets the last node of the path.

       ///Sets the last node of the path. If the path is empty, this

       ///function or setFrom() have to be called before any call of \ref

       ///pushFront() or \ref pushBack()

       void setTo(const GraphNode &) {}

       ///\sa setTo

       ///\sa setStart

       ///\sa setFrom

       ///\sa setStart

 //       ///Desctuctor

       // FIXME: Hmm, pontosan hogy is kene ezt csinalni?

       // Hogy kenyelmes egy ilyet hasznalni?

 //       ///The desctuctor.

       void reserve(size_t r) {

 //       ///It commit also commit the changes.

 	front.reserve(r);

 //       ///\todo Is this what we want?

 	back.reserve(r);

 //  Nope. Let's use commit() explicitly.

       }

 //       ~Builder() { commit(); }

     private:

       bool empty() {

 	return front.empty() && back.empty() && P.empty();

       }

       GraphNode from() const {

 	if( ! front.empty() )

 	  return P.gr->tail(front[front.size()-1]);

 	else if( ! P.empty() )

 	  return P.gr->tail(P.edges[0]);

 	else if( ! back.empty() )

 	  return P.gr->tail(back[0]);

 	else

 	  return INVALID;

       }

       GraphNode to() const {

 	if( ! back.empty() )

 	  return P.gr->head(back[back.size()-1]);

 	else if( ! P.empty() )

 	  return P.gr->head(P.edges[P.length()-1]);

 	else if( ! front.empty() )

 	  return P.gr->head(front[0]);

 	else

 	  return INVALID;

       }

     };

   };

   /**********************************************************************/

   //! \brief A structure for representing undirected path in a graph.

   //!

   //! A structure for representing undirected path in a graph. Ie. this is

   //! a path in a \e directed graph but the edges should not be directed

   //! forward.

   //!

   //! \param Graph The graph type in which the path is.

   //! \param DM DebugMode, defaults to DefaultDebugMode.

   //! 

   //! In a sense, the path can be treated as a graph, for is has \c NodeIt

   //! and \c EdgeIt with the same usage. These types converts to the \c Node

   //! and \c Edge of the original graph.

   //!

   //! \todo Thoroughfully check all the range and consistency tests.

   template<typename Graph, typename DM = DefaultDebugMode>

   class UndirPath {

   public:

     typedef typename Graph::Edge GraphEdge;

     typedef typename Graph::Node GraphNode;

     class NodeIt;

     class EdgeIt;

   protected:

     const Graph *gr;

     typedef std::vector<GraphEdge> Container;

     Container edges;

   public:

     /// \param _G The graph in which the path is.

     ///

     UndirPath(const Graph &_G) : gr(&_G) {}

     /// \brief Subpath constructor.

     ///

     /// Subpath defined by two nodes.

     /// \warning It is an error if the two edges are not in order!

     UndirPath(const UndirPath &P, const NodeIt &a, const NodeIt &b) {

       if( DM::range_check && (!a.valid() || !b.valid) ) {

 	// FIXME: this check should be more elaborate...

 	fault("UndirPath, subpath ctor: invalid bounding nodes");

       }

       gr = P.gr;

       edges.insert(edges.end(), P.edges.begin()+a.idx, P.edges.begin()+b.idx);

     }

     /// \brief Subpath constructor.

     ///

     /// Subpath defined by two edges. Contains edges in [a,b)

     /// \warning It is an error if the two edges are not in order!

     UndirPath(const UndirPath &P, const EdgeIt &a, const EdgeIt &b) {

       if( DM::range_check && (!a.valid() || !b.valid) ) {

 	// FIXME: this check should be more elaborate...

 	fault("UndirPath, subpath ctor: invalid bounding nodes");

       }

       gr = P.gr;

       edges.insert(edges.end(), P.edges.begin()+a.idx, P.edges.begin()+b.idx);

     }

     /// Length of the path.

     size_t length() const { return edges.size(); }

     /// Returns whether the path is empty.

     bool empty() const { return edges.empty(); }

     /// Resets the path to an empty path.

     void clear() { edges.clear(); }

     /// \brief Starting point of the path.

     ///

     /// Starting point of the path.

     /// Returns INVALID if the path is empty.

     GraphNode from() const {

       return empty() ? INVALID : gr->tail(edges[0]);

     }

     /// \brief End point of the path.

     ///

     /// End point of the path.

     /// Returns INVALID if the path is empty.

     GraphNode to() const {

       return empty() ? INVALID : gr->head(edges[length()-1]);

     }

     /// \brief Initializes node or edge iterator to point to the first

     /// node or edge.

     ///

     /// \sa nth

     template<typename It>

     It& first(It &i) const { return i=It(*this); }

     /// \brief Initializes node iterator to point to the node of a given index.

     NodeIt& nth(NodeIt &i, int n) const {

       if( DM::range_check && (n<0 || n>int(length())) )

 	fault("UndirPath::nth: index out of range");

       return i=NodeIt(*this, n);

     }

     /// \brief Initializes edge iterator to point to the edge of a given index.

     EdgeIt& nth(EdgeIt &i, int n) const {

       if( DM::range_check && (n<0 || n>=int(length())) )

 	fault("UndirPath::nth: index out of range");

       return i=EdgeIt(*this, n);

     }

     /// Checks validity of a node or edge iterator.

     template<typename It>

     static

     bool valid(const It &i) { return i.valid(); }

     /// Steps the given node or edge iterator.

     template<typename It>

     static

     It& next(It &e) {

       if( DM::range_check && !e.valid() )

 	fault("UndirPath::next() on invalid iterator");

       return ++e;

     }

     /// \brief Returns node iterator pointing to the head node of the

     /// given edge iterator.

     NodeIt head(const EdgeIt& e) const {

       if( DM::range_check && !e.valid() )

 	fault("UndirPath::head() on invalid iterator");

       return NodeIt(*this, e.idx+1);

     }

     /// \brief Returns node iterator pointing to the tail node of the

     /// given edge iterator.

     NodeIt tail(const EdgeIt& e) const {

       if( DM::range_check && !e.valid() )

 	fault("UndirPath::tail() on invalid iterator");

       return NodeIt(*this, e.idx);

     }

     /*** Iterator classes ***/

     class EdgeIt {

       friend class UndirPath;

       int idx;

       const UndirPath *p;

     public:

       EdgeIt() {}

       EdgeIt(Invalid) : idx(-1), p(0) {}

       EdgeIt(const UndirPath &_p, int _idx = 0) :

 	idx(_idx), p(&_p) { validate(); }

       bool valid() const { return idx!=-1; }

       operator GraphEdge () const {

 	return valid() ? p->edges[idx] : INVALID;

       }

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

       bool operator==(const EdgeIt& e) const { return idx==e.idx; }

       bool operator!=(const EdgeIt& e) const { return idx!=e.idx; }

       bool operator<(const EdgeIt& e) const { return idx<e.idx; }

     private:

       // FIXME: comparison between signed and unsigned...

       // Jo ez igy? Vagy esetleg legyen a length() int?

       void validate() { if( size_t(idx) >= p->length() ) idx=-1; }

     };

     class NodeIt {

       friend class UndirPath;

       int idx;

       const UndirPath *p;

     public:

       NodeIt() {}

       NodeIt(Invalid) : idx(-1), p(0) {}

       NodeIt(const UndirPath &_p, int _idx = 0) :

 	idx(_idx), p(&_p) { validate(); }

       bool valid() const { return idx!=-1; }

       operator const GraphNode& () const {

 	if(idx >= p->length())

 	  return p->to();

 	else if(idx >= 0)

 	  return p->gr->tail(p->edges[idx]);

 	else

 	  return INVALID;

       }

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

       bool operator==(const NodeIt& e) const { return idx==e.idx; }

       bool operator!=(const NodeIt& e) const { return idx!=e.idx; }

       bool operator<(const NodeIt& e) const { return idx<e.idx; }

     private:

       void validate() { if( size_t(idx) > p->length() ) idx=-1; }

     };

     friend class Builder;    

     /**

      * \brief Class to build paths

      * 

      * \ingroup datas

      * This class is used to fill a path with edges.

      *

      * You can push new edges to the front and to the back of the path in

      * arbitrary order then you should commit these changes to the graph.

      *

      * Fundamentally, for most "Paths" (classes fulfilling the

      * PathConcept) while the builder is active (after the first modifying

      * operation and until the commit()) the original Path is in a

      * "transitional" state (operations ot it have undefined result). But

      * in the case of UndirPath the original path is unchanged until the

      * commit. However we don't recomend that you use this feature.

      */

     class Builder {

       UndirPath &P;

       Container front, back;

     public:

       ///\param _P the path you want to fill in.

       ///

       Builder(UndirPath &_P) : P(_P) {}

       /// Sets the starting node of the path.

       /// Sets the starting node of the path. Edge added to the path

       /// afterwards have to be incident to this node.

       /// It should be called iff the path is empty and before any call to

       /// \ref pushFront() or \ref pushBack()

       void setStart(const GraphNode &) {}

       ///Push a new edge to the front of the path

       ///Push a new edge to the front of the path.

       ///\sa setStart

       void pushFront(const GraphEdge& e) {

 	if( DM::consistensy_check && !empty() && P.gr->head(e)!=from() ) {

 	  fault("UndirPath::Builder::pushFront: nonincident edge");

 	}

 	front.push_back(e);

       }

       ///Push a new edge to the back of the path

       ///Push a new edge to the back of the path.

       ///\sa setStart

       void pushBack(const GraphEdge& e) {

 	if( DM::consistensy_check && !empty() && P.gr->tail(e)!=to() ) {

 	  fault("UndirPath::Builder::pushBack: nonincident edge");

 	}

 	back.push_back(e);

       }

       ///Commit the changes to the path.

       void commit() {

 	if( !(front.empty() && back.empty()) ) {

 	  Container tmp;

 	  tmp.reserve(front.size()+back.size()+P.length());

 	  tmp.insert(tmp.end(), front.rbegin(), front.rend());

 	  tmp.insert(tmp.end(), P.edges.begin(), P.edges.end());

 	  tmp.insert(tmp.end(), back.begin(), back.end());

 	  P.edges.swap(tmp);

 	  front.clear();

 	  back.clear();

 	}

       }