src/main/java/com/basistech/tclre/ColorMap.java
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