∀ {α : Type u_1} {β : α → Type u_2} (k : α) (v : β k),
  (Std.DTreeMap.Internal.Impl.inner 1 k v Std.DTreeMap.Internal.Impl.leaf Std.DTreeMap.Internal.Impl.leaf).size =
    Std.DTreeMap.Internal.Impl.leaf.size + 1