Счетчик команд как управление в виде данных

(begin
  (define (step state)
    (let ((pc (car state))
          (a (cadr state))
          (b (caddr state))
          (temp (cadddr state))
          (cycles (list-ref state 4)))
      (cond ((= pc 0) (list (if (= b 0) 5 1) a b temp cycles))
            ((= pc 1) (list 2 a b (remainder a b) cycles))
            ((= pc 2) (list 3 b b temp cycles))
            ((= pc 3) (list 4 a temp temp cycles))
            ((= pc 4) (list 0 a b temp (+ cycles 1)))
            (else state))))
  (define (run state)
    (if (= (car state) 5) state (run (step state))))
  (run (list 0 10 4 0 0)))