($ mk-counter
   (-> (init)
       ($ counter (- init 1))
       (-> ()
           (! counter (+ counter 1))
           counter)))

($ a (mk-counter 5))
($ b (mk-counter 32))

(assert= 5 (a))
(assert= 32 (b))
(assert= 33 (b))
(assert= 6 (a))
(assert= 7 (a))
(assert= 34 (b))