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