; 8/9th function
(define (my-delay thunk)
 (mcons 0 (mcons thunk thunk)))

(define (my-force prom)
  (if (= (modulo (mcar prom) 5) 0)
      (begin (set-mcar! prom (+ (mcar prom) 1))
             (set-mcar! (mcdr prom) ((mcdr (mcdr prom))))
             (mcar (mcdr prom)))
      (begin (set-mcar! prom (+ (mcar prom) 1))
             (mcar (mcdr prom)))))