unfold unsublist
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
i : (List.sublist p (x :: xs)).Index
hx : ¬p x
⊢ (unsublist p i).val = i.val.val
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
i : (List.sublist p (x :: xs)).Index
hx : ¬p x
⊢ (if hh : p x then
        let_fun this := ⋯;
        match ⋯ ▸ i with
        | head => head
        | i.tail => (unsublist p i).tail
      else
        let_fun this := ⋯;
        (unsublist p (this ▸ i)).tail).val =
    i.val.val