@Override
	public int compare(Drillable one, Drillable two) {
		Proficiency p1 = one.getPractice() == null ? null : one.getPractice().getProficiency();
		Proficiency p2 = two.getPractice() == null ? null : two.getPractice().getProficiency();
		
		if (p1 == null && p2 == null) {
			return 0;
		}
		if (p1 == null) {
			return -1;
		}
		if (p2 == null) {
			return 1;
		}
		return p1.compareTo(p2);
	}