return "-";
return pfx + "-";
	public String toString() {		switch (kind & ~EXCEPTION) {		case NULL:			return "n";		case WEAK_NULL:			return "w";		case NN:			return "N";		case WEAK_NN:			return "W";		case NSP:			return "s";		case DNR:			return "-";		default:			throw new IllegalStateException("unknown kind of IsNullValue: " + kind);		}	}