A quick Hint that will be needed to solve this:

If you write the appropriate postcondition, find the Weakest Precondition, and find that the Weakest Precondition P is TRUE, that constitues a proof.

Think about: what is the semantics of "swap(x, y)" ? (look at what we did for min(x,y) in class.)

Q 1. Prove that the statement:

If (a>b) then swap(a, b) else null;

arranges a and b in non-decreasing order.

Q 2. (Challenge) Prove that the statements:

If (a>b) then swap(a, b) else null;

If (b>c) then swap(b, c) else null;

If (a>b) then swap(a, b) else null;

arrange a, b and c in non-decreasing order.