simp [List.get?_eq_some]
case a.a
n : Nat
α : Type u_1
f : Fin n → α
i : Nat
x : α
⊢ x ∈ List.get? (ofFn f).data i ↔ x ∈ List.get? (List.map f (List.fins n)) i
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