void examining_node(node_t u, Graph & g){
cout << " examining node " << u->id() << "\n";
}
void discover_node(node_t u, Graph & g){
cout << "discovered node " << u->id() << "\n";
}
auto operator()(const node_t& a, const node_t& b)
{
return _matrix(a.id(), b.id());
}