unfold unsublist
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
⊢ unsublist p (⋯ ▸ head) = head
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
⊢ (if hh : p x then
      let_fun this := ⋯;
      match ⋯ ▸ ⋯ ▸ head with
      | head => head
      | i.tail => (unsublist p i).tail
    else
      let_fun this := ⋯;
      (unsublist p (this ▸ ⋯ ▸ head)).tail) =
    head