∀ {α : Type u} {β : α → Type v} [inst : Ord α] (t : Std.DTreeMap.Internal.Impl α β) (htb : t.Balanced) (a : α)
  (b : β a),
  (Std.DTreeMap.Internal.Impl.containsThenInsert! a b t).2 =
    (Std.DTreeMap.Internal.Impl.containsThenInsert a b t htb).2.impl