An exercise from The Seasoned Schemer. I really like the shape and feel of this.

union.scm

; Given two sets, return a new set containing the unique members from each.
(define (union set1 set2)
  (letrec
    ((union
       (lambda (set)
         (cond
           ((null? set) set2)
           ((member? (car set) set2)
            (union (cdr set)))
           (else
             (cons (car set)
                   (union (cdr set)))))))
     (member?
       (lambda (a set)
         (cond
           ((null? set) #f)
           ((equal? (car set) a) #t)
           (else
             (member? a (cdr set)))))))
    (union set1)))