#lang sicp

(define f
  (let ((x 0))
    (lambda (n)
      (let ((old-x x))
        (begin
          (set! x n)
          (cond ((= n 0)
                 (if (= old-x 0) -1 0))
                (else n)))))))

(define left-to-right
  (let ((left (f 0))
        (right (f 1)))
    (+ left right)))

(define right-to-left
  (let ((right (f 1))
        (left (f 0)))
    (+ left right)))

;; (f 0) first: (+ (f 0) (f 1)) -> 0
;; (f 1) first: (+ (f 0) (f 1)) -> 1