rw [val_ndrec _ (List.sublist_pos hx).symm.symm.symm]
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
⊢ (let_fun this := ⋯;
      head).val =
    (⋯ ▸ head).val.val
no goals