class OutEdgeIt;

     class OutEdgeIt;

     class InEdgeIt;

     class InEdgeIt;

     class SymEdgeIt;

     class SymEdgeIt;

     typedef ListGraph Graph;

     typedef ListGraph Graph;

   private:

   private:

     int node_id;

     int node_id;

     int edge_id;

     int edge_id;

     int _node_num;

     int _node_num;

     int _edge_num;

     int _edge_num;

     node_item* _first_node;

     node_item* _first_node;

     node_item* _last_node;

     node_item* _last_node;

     class node_item {

     class node_item {

       friend class ListGraph;

       friend class ListGraph;