∀ {α : Type u} {β : α → Type v} [inst : Ord α] (k : α) (t : Std.DTreeMap.Internal.Impl α β),
  Std.DTreeMap.Internal.Impl.contains k t =
    match t with
    | Std.DTreeMap.Internal.Impl.leaf => false
    | Std.DTreeMap.Internal.Impl.inner size k' v l r =>
      match compare k k' with
      | Ordering.lt => Std.DTreeMap.Internal.Impl.contains k l
      | Ordering.gt => Std.DTreeMap.Internal.Impl.contains k r
      | Ordering.eq => true