n ≤ 1 + # x0, n - 1 ≤ # x0 → x0 = y + z ∧ # y = n - 1 ⊦ ∃ y, z. a ' + x0 = y + z ∧ # y = n
Used simplifier rules: ⊦ n -1 = n - 1; (s)