\li attributes

 The attributes section can handle some information about the graph. It

 contains in each line an key and the mapped value to key. The key should

 be a string without whitespace, the value can be from various type.

 \code

 @attributes

 title "Four colored plan graph"

 author "Balazs DEZSO"

 copyright "Lemon Library"

 version 12

 \endcode

 \code

 @end

 \endcode

 =======

 \endcode

 With \c writeAttribute() function you can write an attribute to the file.

 \code

 writer.writeAttribute("author", "Balazs DEZSO");

 writer.writeAttribute("version", 12);

 \endcode

 With \c readAttribute() function you can read an attribute from the file.

 \code

 std::string author;

 writer.readAttribute("author", author);

 int version;

 writer.writeAttribute("version", version);

 \section undir Undir graphs

 In the undir graph format there is an \c undiredgeset section instead of

 the \c edgeset section. The first line of the section describes the 

 undirected egdes' names and all next lines describes one undirected edge

 with the the incident nodes and the values of the map.

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

 is two map which names are the same with a \c '+' and a \c '-' prefix

 then it can be read as an directed map.

 \code

 @undiredgeset

              id    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 changed to \c undiredges section. This section

 describes labeled edges and undirected edges. The directed edge label

 should start with a \c '+' and a \c '-' prefix what decide the direction

 of the edge. 

 \code

 @undiredges

 undiredge 1

 +edge 5

 -back 5

 \endcode

 There are similar classes to the \c GraphReader ans \c GraphWriter

 which handle the undirected graphs. These classes are the 

 \c UndirGraphReader and \UndirGraphWriter.

 The \c readUndirMap() function reads an undirected map and the

 \c readUndirEdge() reads an undirected edge from the file, 

 \code

 reader.readUndirEdgeMap("capacity", capacityMap);

 reader.readEdgeMap("flow", flowMap);

 ...

 reader.readUndirEdge("undir_edge", undir_edge);

 reader.readEdge("edge", edge);

 \endcode

 \section advanced Advanced features

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

 graphs. But sometimes we want more advanced features. This way we can

 use the more general lemon reader and writer interface.

 The lemon format is an section oriented file format. It contains one or

 more section, each starts with a line with \c \@ first 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 \c LemonReader and \c LemonWriter gives a framework to read and

 write sections. There are various section reader and section writer

 classes which can be attached to a \c LemonReader or a \c LemonWriter.

 There are default section readers and writers for reading and writing

 item sets, and labeled items in the graph. These reads and writes

 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 \c 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 

 by example as a network traffic matrix.

 In 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 an directed NewEdgeSetAdaptor class. The example

 shows the input with the LemonReader class:

 \code

 UndirListGraph network;

 UndirListGraph::UndirEdgeSet<double> capacity;

 NewEdgeSetAdaptor<UndirListGraph> traffic(network);

 NewEdgeSetAdaptor<UndirListGraph>::EdgeSet<double> request(network);

 LemonReader reader(std::cin);

 NodeSetReader nodesetReader(reader, network);

 UndirEdgeSetReader undirEdgesetReader(reader, network, nodesetReader);

 undirEdgesetReader.readEdgeMap("capacity", capacity);

 EdgeSetReader edgesetReader(reader, traffic, nodesetReader);

 edgesetReader.readEdgeMap("request", request);

 reader.run();

 \endcode

 Because the GraphReader and the UndirGraphReader can be converted

 to LemonReader and it can resolve the ID's of the items, the previous

 result can be achived with the UndirGraphReader class also.

 \code

 UndirListGraph network;

 UndirListGraph::UndirEdgeSet<double> capacity;

 NewEdgeSetAdaptor<UndirListGraph> traffic(network);

 NewEdgeSetAdaptor<UndirListGraph>::EdgeSet<double> request(network);

 UndirGraphReader reader(std::cin, network);

 reader.readEdgeMap("capacity", capacity);

 EdgeSetReader edgesetReader(reader, traffic, reader);

 edgesetReader.readEdgeMap("request", request);

 reader.run();

 \endcode