;; You may not have implemented <= . If you haven't,
;; add this to the top:
;; (define <=
;;   (lambda (x y)
;;     (not (> x y))))

(define A
  (lambda (k x1 x2 x3 x4 x5)
    (letrec ((B
              (lambda ()
                (begin
                  (set! k (- k 1))
                  (A k B x1 x2 x3 x4)))))
      (if (<= k 0)
          (+ (x4) (x5))
          (B)))))

(A 10 (lambda () 1) (lambda () -1) (lambda () -1) (lambda () 1) (lambda () 0))