#| BEGIN (Write your solution here) |#
(define (adjoin-set x set)
  (if (null? set)
      (cons x set)
      (let ((head (car set))
            (tail (cdr set)))
        (cond ((= x head) set)
              ((> x head) (cons head (adjoin-set x tail)))
              (else (cons x set))))))
#| END |#