for (int i=0; i<n; i++)
                            for (int j=i+1; j<n; j++) {
                                graph_traits<Graph>::out_edge_iterator e_start, e_end,
                                    e_start2, e_end2;;
                                boost::tie(e_start, e_end) = boost::edge_range(i,j,g);
                                // check if an edge exists
                                if (e_start == e_end) continue;
                                boost::tie(e_start2, e_end2) = boost::edge_range(j,i,g);
                                // set reverse edge, set its capacity to be zero
                                g[*e_start].rev = *(--e_end2);
                                g[*e_start2].rev = *(--e_end);
                                g[*e_start].capacity = xval[i][j];
                                g[*e_start].res_capacity = xval[i][j];
                                g[*e_start2].capacity = xval[i][j];
                                g[*e_start2].res_capacity = xval[i][j];
                                // now end points to the second parallel edge
                                g[*e_end].capacity = 0;
                                g[*e_end].res_capacity = 0;
                                g[*e_end2].capacity = 0;
                                g[*e_end2].res_capacity = 0;
                            }