∀ {α : Type u_1} {β : α → Type u_2} [inst : Ord α] (k : α → Ordering) (size : ℕ) (k' : α) (v' : β k')
  (l r : Std.DTreeMap.Internal.Impl α β),
  k k' = Ordering.lt →
    Std.DTreeMap.Internal.Impl.contains' k (Std.DTreeMap.Internal.Impl.inner size k' v' l r) = true →
      Std.DTreeMap.Internal.Impl.contains' k l = true