Exercise 2.65.  Use the results of exercises 2.63 and 2.64 to give ϴ(n) 
implementations of union-set and intersection-set for sets implemented 
as (balanced) binary trees.

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I'm sure this could be done more efficiently.

(define (union-set a b)
  (list->tree (union-list (tree->list-2 a)
                          (tree->list-2 b))))

(define (union-list a b)
  (cond ((null? a) b)
        ((null? b) a)
        ((= (car a) (car b)) (cons (car a) (union-list (cdr a) (cdr b))))
        ((< (car a) (car b)) (cons (car a) (union-list (cdr a) b))))
        (else                (cons (car b) (union-list a (cdr b)))))

(define (intersection-set a b)
  (list->tree (intersection-list (tree->list-2 a)
                                 (tree->list-2 b))))

(define (intersection-list a b)
  (cond ((or (null? a) (null? b)) '())
        ((= (car a) (car b)) (cons (car a) (intersection-list (cdr a) (cdr b))))
        ((< (car a) (car b)) (intersection-list (cdr a) b))
        (else (intersection-list a (cdr b)))))