⊦ m + n = n + m
n ≤ n0, n1 ≤ m0 + 1, m0 + n0 + 1 < n + n1, ∀ n, n0, n1. n ≤ n0 ∧ n1 ≤ m0 → n + n1 ≤ n0 + m0 ⊦ n + n1 = 0
Used terms: n0, 1