(mod (X)
  (include *standard-cl-23*)

  (defun S (C)
    (if C
      (assign-lambda 
        Z (f (f C))
        Y (r (f C))
        R (r C)
        (if (= Z 2)
          (if (= 1 (f Y))
            (r Y)
            (S R)
            )
          (S R)
          )
        )
        ()
      )
    )

  (S X)
  )