passing an alist to a procedure without making a copy?

[Mark Polesky]

(define my-alist
  '((a . 1)
    ))
    
(set! my-alist (acons 'b 2 my-alist))

my-alist ==> ((b . 2) (a . 1))

(define (alist-prepend alist key value)
  (set! alist (acons key value alist)))

(alist-prepend my-alist 'c 3)

my-alist ==> ((b . 2) (a . 1))

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How can I get (alist-prepend) to operate
on the original alist?

Thanks,
Mark