Swapping for (mutable) reference types:

Statement	Variable Values
private static void swap3(NaturalNumber n1, NaturalNumber n2) {
	n1 n2
NaturalNumber tmp = n1;
	n1, tmp n2
n1 = n2;
	n1, n2 tmp
n2 = tmp;
	n1 n2, tmp
}
Start tracing here
NaturalNumber x = new NaturalNumber2(41), y = new NaturalNumber2(78);
	x y
swap3(x, y);
	x y