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