rfl
α : Type u_1
p✝¹ : α → Prop
inst✝² : DecidablePred p✝¹
p✝ : α → Prop
inst✝¹ : DecidablePred p✝
p : α → Prop
inst✝ : DecidablePred p
x : α
xs : List α
h : p x
x✝ : (⟨x, h⟩ :: List.sublist p xs).Index
heq✝ : ⋯ ▸ ⋯ ▸ head = head
⊢ (let_fun this := ⋯;
    head) =
    head
no goals