apply ih
case refl
α : Type u_1
p✝ : α → Prop
inst✝¹ : DecidablePred p✝
p : α → Prop
inst✝ : DecidablePred p
x : α
xs : List α
ih : ∀ (i : (List.sublist p xs).Index), (unsublist p i).val = i.val.val
hx : p x
x✝ : (⟨x, hx⟩ :: List.sublist p xs).Index
i✝ : (List.sublist p xs).Index
⊢ (unsublist p i✝).val = i✝.val.val
no goals