(define (make-counter) (let ((var 0)) (lambda () (set! var (+ 1 var)))))

(define counter1 (make-counter))

(assert (= (counter1) 1))

(define counter2 (make-counter))

(assert (= (counter2) 1))

(assert (= (counter2) 2))

(assert (= (counter1) 2))

(assert (= (counter1) 3))

(assert (= (counter2) 3))

(assert (= (counter2) 4))

(assert (= (counter2) 5))

(assert (= (counter1) 4))

(assert (= (counter1) 5))

(define (g) (define x 0) (lambda () (define temp x) (mutate x (+ x 1)) temp))

(define (make-key-counter) ((lambda (:keys var) (lambda () (mutate var (+ 1 var)) var)) :var 0))

(define key-counter1 (make-key-counter))

(assert (= (key-counter1) 1))

(define key-counter2 (make-key-counter))

(assert (= (key-counter2) 1))

(assert (= (key-counter2) 2))

(assert (= (key-counter1) 2))

(assert (= (key-counter1) 3))

(assert (= (key-counter2) 3))

(assert (= (key-counter2) 4))

(assert (= (key-counter2) 5))

(assert (= (key-counter1) 4))

(assert (= (key-counter1) 5))