;; A set module, based on binary trees (from sicp)

(define (entry tree) (car tree))
(define (left-branch tree) (cadr tree))
(define (right-branch tree) (caddr tree))

(define (make-tree entry left right)
  (list entry left right))

(define (element-of-set? x set)
  (cond ((null? set) #f)
        ((= x (entry set)) #t)
        ((< x (entry set))
         (element-of-set? x (left-branch set)))
        ((> x (entry set))
         (element-of-set? x (right-branch set)))))

(define (adjoin-set x set)
  (cond ((null? set) (make-tree x '() '()))
        ((= x (entry set)) set)
        ((< x (entry set))
         (make-tree (entry set)
                    (adjoin-set x (left-branch set))
                    (right-branch set)))
        ((> x (entry set))
         (make-tree (entry set)
                    (left-branch set)
                    (adjoin-set x (right-branch set))))))

(define (intersection-set set1 set2)
  (if (or (null? set1) (null? set2))
    '()
    (let ((x1 (car set1))
          (x2 (car set2)))
      (cond ((= x1 x2)
             (cons x1
                   (intersection-set (cdr set1)
                                     (cdr set2))))
            ((< x1 x2)
             (intersection-set (cdr set1) set2))
            ((< x2 x1)
             (intersection-set set1 (cdr set2)))))))