∀ {α : Type u_1} {n : ℕ} {f : Fin n → α} (i : ℕ) (a : α),
  ((List.ofFn f)[i]? = some a) = ¬(List.pmap (fun i hi => f ⟨i, hi⟩) (List.range n) ⋯)[i]? = some a →
    ¬(List.ofFn f)[i]? = some a → i < (List.range n).length