m * n2 = m * n1 + n3, n2 < n1 ⊦ m * (n2 - n1) = n3
Used simplifier rules: ⊦ m < n → n ≠ 0; (s)%f: ⊦ m < n → n ≠ 0; (forward) from specification nat-basic% ⊦ m ≤ n ↔ ¬ n < m; (ss)