⊦ encode(m, n) = ((m + n) * (m + n + 1)) / 2 + n
⊦ ((m + n) * (m + n + 1)) / 2 + n = ((m0 + n0) * (m0 + n0 + 1)) / 2 + n0 ↔ m = m0 ∧ n = n0
Used terms: n0, m0