($ 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))