(*os) << " " << _source - _vertices << " [peripheries=2];" << endl;
(*os) << " " << _sink - _vertices << " [peripheries=2];" << endl;
for(int k = 0; k < _nb_edges; k++) {
(*os) << " " << _source - _vertices << " [peripheries=2];" << endl;
(*os) << " " << _sink - _vertices << " [peripheries=2];" << endl;
for(int k = 0; k < _nb_edges; k++) {