(let car (get-user-car)
    (map1 color-wheel car!wheels))

That's a good argument. I can see the havoc that would result.

Still, I'm willing to risk that so that I have the option to do something like:

  (with (ocar car car-counter 0)
    (let car (fn (xs) (++ car-counter) (ocar xs))
      (map1 color-wheel car!wheels))
    (prn "map1 called car " car-counter " times"))