public Mark next() {
		Mark next;
		switch (this) {
			case RED:
				next = YELLOW;
				break;
			case YELLOW:
				next = GREEN;
				break;
			case GREEN:
				next = BLUE;
				break;
			case BLUE:
				next = RED;
				break;
			default:
				next = EMPTY;
				break;
		}
		return next;
	}