/// writes the node section and the first arc section, then the

   /// writes the node section and the first arc section, then the

   /// second pass skips the node section and writes just the arc

   /// second pass skips the node section and writes just the arc

   /// section to the stream. The output stream can be retrieved with

   /// section to the stream. The output stream can be retrieved with

   /// the \c ostream() function, hence the second pass can append its

   /// the \c ostream() function, hence the second pass can append its

   /// output to the output of the first pass.

   /// output to the output of the first pass.

   class DigraphWriter {

   class DigraphWriter {

   public:

   public:

     TEMPLATE_DIGRAPH_TYPEDEFS(Digraph);

     TEMPLATE_DIGRAPH_TYPEDEFS(Digraph);

   private:

   private:

   /// The arc maps are written into the file as two columns, the

   /// The arc maps are written into the file as two columns, the

   /// caption of the columns are the name of the map prefixed with \c

   /// caption of the columns are the name of the map prefixed with \c

   /// '+' and \c '-'. The arcs are written into the \c \@attributes

   /// '+' and \c '-'. The arcs are written into the \c \@attributes

   /// section as a \c '+' or a \c '-' prefix (depends on the direction

   /// section as a \c '+' or a \c '-' prefix (depends on the direction

   /// of the arc) and the label of corresponding edge.

   /// of the arc) and the label of corresponding edge.

   class GraphWriter {

   class GraphWriter {

   public:

   public:

     TEMPLATE_GRAPH_TYPEDEFS(Graph);

     TEMPLATE_GRAPH_TYPEDEFS(Graph);

   private:

   private:

       }

       }

     }

     }

   private:

   private:

     GraphWriter(GraphWriter& other)

     GraphWriter(GraphWriter& other)

       : _os(other._os), local_os(other.local_os), _graph(other._graph),

       : _os(other._os), local_os(other.local_os), _graph(other._graph),

         _skip_nodes(other._skip_nodes), _skip_edges(other._skip_edges) {

         _skip_nodes(other._skip_nodes), _skip_edges(other._skip_edges) {