#| BEGIN (Введите свое решение) |#
(define (element-of-set? x set)
  (cond ((null? set) #f)
        ((equal? x (car set)) #t)
        (else (element-of-set? x (cdr set)))))

(define (union-set set1 set2)
  (cond ((null? set2) set1)
        ((element-of-set? (car set2) set1)
         (union-set set1 (cdr set1)))
        (else (union-set (cons (car set2) set1) (cdr set2)))))
#| END |#