[plt-scheme] Meaning of set-box!

My understanding is that set-box! is used to modify the contents of a box, but neither of these seem to work

> (define (my-set-bang x a)
    (set-box! x a))
> (define x 1)
> (my-set-bang (box x) 2)
> x
1
> (set-box! (box x) 2)
> x
1

On the other hand, I can see that the box contents does change, even if the variable x does not change

> (define x 1)
> (define y (box x))
> (set-box! y 2)
> x
1
> (unbox y)
2
> 

What I'm trying to do is define a mutator. to be specific, I want to make incremental updates to an environment/symbol table. What I'm doing now is something like

(define (evaluate exp)
  lambda (env)
  ...
  (values result new-env))

===
Gregory Woodhouse  <gregory.woodhouse at va.gov <mailto:gregory.woodhouse at med.va.gov> >
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