Proof step 6 from proof append-from-le (previous step)
n0 ≤ # x0, ∀ x0, a, y. n0 ≤ # x0 + 1 ∧ (∀ n, y. n ≤ # x0 → x0 + y from n = (x0 from n) + y) → (a ' + x0) + y from n0 = (a ' + x0 from n0) + y,
∀ n, y. n ≤ # x0 → x0 + y from n = (x0 from n) + y
⊦ x0 + y from n0 = (x0 from n0) + y
Heuristic quantifier closing applied rule all left