∀ {α : Type u} {cmp : α → α → Ordering} [inst : Std.TransCmp cmp] [inst_1 : BEq α] [Std.LawfulBEqCmp cmp]
  [inst_3 : Inhabited α] {l : List α} {k : α}, l.contains k = false → (Std.ExtTreeSet.ofList l cmp).get! k = default