m0 + n = m0 + n0, ∀ n, n0. m0 + n = m0 + n0 → n = n0 ⊦ n = n0
Used simplifier rules: rec: ⊦ m +1 + n = (m + n)+1;%inj-1: ⊦ n +1 = n0 +1 ↔ n = n0; from specification nat-basic1