Exercise 2.5.  Show that we can represent pairs of nonnegative integers using 
only numbers and arithmetic operations if we represent the pair a and b as the 
integer that is the product 2^a 3^b. Give the corresponding definitions of the 
procedures cons, car, and cdr.

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Since 2 and 3 are relatively prime, we can simply factor the product, and 
count the occurrences of 2 and 3 in the factorization to retrieve a and b.

(define (cons a b)
  (* (expt 2 a) (expt 3 b)))

(define (helper n factor accum)
  (if (= (modulo n factor) 0)
      accum
      (helper (/ n factor) (+ accum 1))))

(define (car z)
  (helper z 2 0))

(define (cdr z)
  (helper z 3 0))