@Override
    public String toString() {
        switch(this) {
            case A: return "A";
            case B: return "B";
            case C: return "C";
            case D: return "D";
            case BE: return "B+E";
            case CE: return "C+E";
            case DE: return "D+E";
            default: return null; // unreachable
        }
    }