2.61.ss

(define (element-of-set? x set)
  (cond ((null? set) #f)
        ((= x (car set)) #t)
        ((< x (car set)) #f)
        (else (element-of-set? x (cdr set)))))

(define (adjoin-set x set)
  (if (null? set)
      (list x)
      (let ((head (car set)))
         (cond ((= x head) set)
               ((< x head) (cons x set))
               ((> x head) (cons head (adjoin-set x (cdr set))))))))