<i>You can&#x27;t change the chair because it&#x27;s all immutable, so you create a new chair along with a new universe and build a new car with the new chair.</i><p>No, if you read Okasaki, you knew you only have to reconstruct the part of the universe (up to its origin) that refers (directly or indirectly) to the seat ;).<p>In fact, you can safely have two universes with different seats, where most of the universes is shared.