n < m ⊦ ⌜n⌝ + n∞1 = ⌜m⌝ + n∞2 ↔ n∞1 = ⌜m - n⌝ + n∞2
Used simplifier rules: ⊦ m < n → n ≠ 0; (s)%f: ⊦ m < n → n ≠ 0; (forward) from specification nat-basic