Theorem 2: Commutativity of addition: 
Proof: Base case: By lemma 1,
.
- Inductive hypothesis: Suppose that, for
,
. - By axiom 6,
. - By theorem 1,
. - By the inductive hypothesis,
. - By theorem 1,
. - By lemma 1,
. - By theorem 1,
. - By axiom 6,
. - By induction,
. QED.