// Copy the graph

       // Copy the graph

       int i = 0;

       int i = 0;

       for (NodeIt n(_graph); n != INVALID; ++n, ++i) {

       for (NodeIt n(_graph); n != INVALID; ++n, ++i) {

         _node_id[n] = i;

         _node_id[n] = i;

       }

       }

         // Store the arcs in a mixed order

         // Store the arcs in a mixed order

         const int skip = std::max(_arc_num / _node_num, 3);

         const int skip = std::max(_arc_num / _node_num, 3);

         int i = 0, j = 0;

         int i = 0, j = 0;

         for (ArcIt a(_graph); a != INVALID; ++a) {

         for (ArcIt a(_graph); a != INVALID; ++a) {

           _arc_id[a] = i;

           _arc_id[a] = i;