namespace lemon {

 /*!

 \page read_write_bg Background of Reading and Writing

 To read a map (on the nodes or edges)

 the \ref lemon::GraphReader "GraphReader"

 should know how to read a Value from the given map.

 By the default implementation the input operator reads a value from

 the stream and the type of the read value is the value type of the given map.

 When the reader should skip a value in the stream, because you do not

 want to store it in a map, the reader skips a character sequence without 

 whitespaces. 

 If you want to change the functionality of the reader, you can use

 template parameters to specialize it. When you give a reading

 command for a map you can give a Reader type as template parameter.

 With this template parameter you can control how the Reader reads

 a value from the stream.

 The reader has the next structure: 

 \code

 struct TypeReader {

   typedef TypeName Value;

   void read(std::istream& is, Value& value);

 };

 \endcode

 For example, the \c "strings" nodemap contains strings and you do not need

 the value of the string just the length. Then you can implement an own Reader

 struct.

 \code

 struct LengthReader {

   typedef int Value;

   void read(std::istream& is, Value& value) {

     std::string tmp;

     is >> tmp;

     value = tmp.length();

   }

 };

 ...

 reader.readNodeMap<LengthReader>("strings", lengthMap);

 \endcode  

 The global functionality of the reader class can be changed by giving a

 special template parameter to the GraphReader class. By default, the

 template parameter is \c DefaultReaderTraits. A reader traits class 

 should provide a nested template class Reader for each type, and a 

 DefaultReader for skipping a value.

 The specialization of writing is very similar to that of reading.

 \section u Undirected graphs

 In a file describing an undirected graph (ugraph, for short) you find an

 \c uedgeset section instead of the \c edgeset section. The first line of

 the section describes the names of the maps on the undirected egdes and all

 next lines describe one undirected edge with the the incident nodes and the

 values of the map.

 The format handles directed edge maps as a syntactical sugar???, if there

 are two maps with names being the same with a \c '+' and a \c '-' prefix

 then this will be read as a directed map.

 \code

 @uedgeset

              label      capacity        +flow   -flow

 32   2       1          4.3             2.0     0.0

 21   21      5          2.6             0.0     2.6

 21   12      8          3.4             0.0     0.0

 \endcode

 The \c edges section is changed to \c uedges section. This section

 describes labeled edges and undirected edges. The directed edge label

 should start with a \c '+' or a \c '-' prefix to decide the direction

 of the edge. 

 \code

 @uedges

 uedge 1

 +edge 5

 -back 5

 \endcode

 There are similar classes to the \ref lemon::GraphReader "GraphReader" and

 \ref lemon::GraphWriter "GraphWriter" which

 handle the undirected graphs. These classes are

 the \ref lemon::UGraphReader "UGraphReader"

 and \ref lemon::UGraphWriter "UGraphWriter".

 The \ref lemon::UGraphReader::readUEdgeMap() "readUEdgeMap()"

 function reads an undirected map and the

 \ref lemon::UGraphReader::readUEdge() "readUEdge()"

 reads an undirected edge from the file, 

 \code

 reader.readUEdgeMap("capacity", capacityMap);

 reader.readEdgeMap("flow", flowMap);

 ...

 reader.readUEdge("u_edge", u_edge);

 reader.readEdge("edge", edge);

 \endcode

 \section advanced Advanced features

 The graph reader and writer classes give an easy way to read and write

 graphs. But sometimes we want more advanced features. In this case we can

 use the more general <tt>lemon reader and writer</tt> interface.

 The LEMON file format is a section oriented file format. It contains one or

 more sections, each starting with a line identifying its type 

 (the word starting with the \c \@  character).

 The content of the section this way cannot contain line with \c \@ first

 character. The file may contains comment lines with \c # first character.

 The \ref lemon::LemonReader "LemonReader"

 and \ref lemon::LemonWriter "LemonWriter"

 gives a framework to read and

 write sections. There are various section reader and section writer

 classes which can be attached to a \ref lemon::LemonReader "LemonReader"

 or a \ref lemon::LemonWriter "LemonWriter".

 There are default section readers and writers for reading and writing

 item sets, and labeled items in the graph. These read and write

 the format described above. Other type of data can be handled with own

 section reader and writer classes which are inherited from the

 \c LemonReader::SectionReader or the

 \ref lemon::LemonWriter::SectionWriter "LemonWriter::SectionWriter"

 classes.

 The next example defines a special section reader which reads the

 \c \@description sections into a string:

 \code 

 class DescriptionReader : LemonReader::SectionReader {

 protected:

   virtual bool header(const std::string& line) {

     std::istringstream ls(line);

     std::string head;

     ls >> head;

     return head == "@description";

   }

   virtual void read(std::istream& is) {

     std::string line;

     while (getline(is, line)) {

       desc += line;

     }

   }

 public:

   typedef LemonReader::SectionReader Parent;

   DescriptionReader(LemonReader& reader) : Parent(reader) {}

   const std::string& description() const {

     return description;

   }

 private:

   std::string desc;

 };

 \endcode

 The other advanced stuff of the generalized file format is that 

 multiple edgesets can be stored to the same nodeset. It can be used 

 for example as a network traffic matrix.

 In our example there is a network with symmetric links and there are assymetric

 traffic request on the network. This construction can be stored in an

 undirected graph and in a directed \c ListEdgeSet class. The example

 shows the input with the \ref lemon::LemonReader "LemonReader" class:

 \code

 ListUGraph network;

 ListUGraph::UEdgeMap<double> capacity;

 ListEdgeSet<ListUGraph> traffic(network);

 ListEdgeSet<ListUGraph>::EdgeMap<double> request(network);

 LemonReader reader(std::cin);

 NodeSetReader<ListUGraph> nodesetReader(reader, network);

 UEdgeSetReader<ListUGraph> 

   uEdgesetReader(reader, network, nodesetReader);

 uEdgesetReader.readEdgeMap("capacity", capacity);

 EdgeSetReader<ListEdgeSet<ListUGraph> > 

   edgesetReader(reader, traffic, nodesetReader, "traffic");

 edgesetReader.readEdgeMap("request", request);

 reader.run();

 \endcode

 Because both the \ref lemon::GraphReader "GraphReader"

 and the \ref lemon::UGraphReader "UGraphReader" can be converted

 to \ref lemon::LemonReader "LemonReader"

 and it can resolve the label's of the items, the previous

 result can be achived with the \ref lemon::UGraphReader "UGraphReader"

 class, too.

 \code

 ListUGraph network;

 ListUGraph::UEdgeSet<double> capacity;

 ListEdgeSet<ListUGraph> traffic(network);

 ListEdgeSet<ListUGraph>::EdgeMap<double> request(network);

 UGraphReader<ListUGraph> reader(std::cin, network);

 reader.readEdgeMap("capacity", capacity);

 EdgeSetReader<ListEdgeSet<ListUGraph> > 

   edgesetReader(reader, traffic, reader, "traffic");

 edgesetReader.readEdgeMap("request", request);

 reader.run();

 \endcode

 \author Balazs Dezso

 */

 }