(define (free-vars xs)
  (define (notin? var list)
    (cond[(null? list) #t]
         [(eq? var (car list)) #f]
         [else (notin? var (cdr list))]))
  (define (rek xs l)
    (cond [(var? xs) (if (notin? xs l)
                         (cons xs l)
                         l)]
          [(neg? xs) (rek (neg-subf xs) l)]
          [(conj? xs) (rek (conj-left xs) (rek (conj-right xs) l))]
          [(disj? xs) (rek (disj-left xs) (rek (disj-right xs) l))]))
  (rek xs '()))