功能限制：

• 使用 Racket BSL
• 不允许使用逻辑运算符
• 只能使用 if、参数名称、#true 和 #false（尽管可能不需要全部）
• 覆盖测试所有可能的参数输入组合
• 不允许使用以下形式的 if（如果参数 #true #false）

这是我迄今为止尝试过的（但我使用了不允许的逻辑运算符（and，not，or）：

; same? : Boolean Boolean -> Boolean
; returns true if either both boolean
; parameters are true or both are false

(check-expect (same? #t #t) #t)
(check-expect (same? #t #f) #f)
(check-expect (same? #f #t) #f)
(check-expect (same? #f #f) #t)

(define (same? x y)
  (or (and x y) (not (or x y))))

; TODO: Design the function non-agreement? that takes two Boolean parameters
;       and returns true if at least one of them is false.

; non-agreement? : Boolean Boolean -> Boolean
; returns true if at least one of the boolean
; parameters is false

(check-expect (non-agreement? #t #t) #f)
(check-expect (non-agreement? #t #f) #t)
(check-expect (non-agreement? #f #t) #t)
(check-expect (non-agreement? #f #f) #t)

(define (non-agreement? x y)
  (not (and x y))

; TODO: Design the function follow-directions that takes two Boolean parameters:
;       * if the first is false, it simply returns the second;
;       * if the first is true, it returns the opposite of the second

; follow-directions : Boolean Boolean -> Boolean
; returns the second boolean parameter if the first
; is false; otherwise, returns the opposite of the
; second boolean parameter if the first is true

(check-expect (follow-directions #t #t) #f)
(check-expect (follow-directions #t #f) #t)
(check-expect (follow-directions #f #t) #t)
(check-expect (follow-directions #f #f) #f)

(define (follow-directions x y)
  (or (and x (not y)) (and (not x) y)))

(define (same? x y)
  (if x
      y  ; = (if y #true #false)
      (if y 
          x ; = #false
          #true)))

(define (non-agreement? x y)
  (if x
      (if y 
          #false
          x) ; = #true
      #true))

(define (follow-directions x y)
  (if x
      (if y
          #false
          x) ; = #true
      y))