Exercise 2.62.  Give a ϴ(n) implementation of union-set for sets 
represented as ordered lists.

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(define (union-set a b)
  (cond ((null? a) b)
        ((null? b) a)
        ((= (car a) (car b)) (cons (car a) (union-set (cdr a) (cdr b))))
        ((> (car a) (car b)) (cons (car b) (union-set a (cdr b))))
        (else (cons (car a) (union-set (cdr a) b)))))