Exercise 2.59.  Implement the union-set operation for the unordered-list 
representation of sets.

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(define (union-set a b)
  (cond ((null? b) a)
        ((element-of-set? (car b) a) (union-set a (cdr b)))
        (else (union-set (adjoin-set (car b) a) (cdr b)))))