void colorchain(Arc a) {
        ColorDesc cd = colorDescs.get(a.co);
        a.colorchain = cd.arcs;
        cd.arcs = a;
    }
void colorchain ( Arc a ) { ColorDesc cd = colorDescs . get ( a . co ) ; a . colorchain = cd . arcs ; cd . arcs = a ; }
colorchain - add this arc to the color chain of its color