ordered-set.scm

(define (element-of-set? x set)
  (cond ((null? set) false)
        ((= x (car set)) true)
        ((< x (car set)) false)
        (else (element-of-set? x (cdr 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)))))))

(define (adjoin-set x set)
    (cond ((null? set) (list x))
          ((<= x (car set)) (cons x set))
          (else (cons (car set) (adjoin-set x (cdr set))))
    )
)

(define (union-set set1 set2)
    (cond ((null? set1) set2)
          ((null? set2) set1)
          (else (let (
              (x1 (car set1))
              (x2 (car set2))
                     )
                (cond ((< x1 x2) (cons x1 (union-set (cdr set1) set2)))
                      ((= x1 x2) (cons x1 (union-set (cdr set1) (cdr set2))))
                      (else (cons x2 (union-set set1 (cdr set2))))
                )
                )
          )
    )
)

(union-set (list 1 2 3 4 5 10 11 20 21 22) (list 3 4 5 6 7 8 9 12 23 24))