(union x y ) returns a list containing the elements that occur in either x or y.

(define (union x y)
  (if (pair? x)
      (if (memv (car x) y)
          (union (cdr x) y)
          (cons (car x) (union (cdr x) y)))
      y))

> (union '(c a t) '(b a g))
(c t b a g)

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