simp only [← Array.getElem_eq_data_get, Array.getElem_ofFn]
case a.a
n : Nat
α : Type u_1
f : Fin n → α
i : Nat
x : α
⊢ (∃ h, List.get (ofFn f).data { val := i, isLt := h } = x) ↔ ∃ a, (∃ h, { val := i, isLt := ⋯ } = a) ∧ f a = x
case a.a
n : Nat
α : Type u_1
f : Fin n → α
i : Nat
x : α
⊢ (∃ h, f { val := i, isLt := ⋯ } = x) ↔ ∃ a, (∃ h, { val := i, isLt := ⋯ } = a) ∧ f a = x