Exercise 4.35.  Write a procedure an-integer-between that returns an 
integer between two given bounds. This can be used to implement a 
procedure that finds Pythagorean triples, i.e., triples of integers 
(i,j,k) between the given bounds such that i < j and i2 + j2 = k2, as 
follows:

(define (a-pythagorean-triple-between low high)
  (let ((i (an-integer-between low high)))
    (let ((j (an-integer-between i high)))
      (let ((k (an-integer-between j high)))
        (require (= (+ (* i i) (* j j)) (* k k)))
        (list i j k)))))

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(define (an-integer-between low high)
  (if (< low high)
      (amb low (an-integer-between (+ low 1) high))
      (amb)))