private HashSet<Integer> getForbiddenColors(Value n) {
        HashSet<Value> edges = adjacencyList.get(n);
        HashSet<Integer> rval = new HashSet<Integer>();
        for(Value v : edges) {
            Integer c = v.getColor();
            if(c != null && c != -1) {
                rval.add(c);
            }
        }
        return rval;
    }