(define (qs elements)
  (if (stream-empty? elements)
      empty-stream
      (let* ([pivot (stream-first elements)]
             [rest (stream-rest elements)]
             [<pivot (λ (x) (< x pivot))]
             [>=pivot (λ (x) (>= x pivot))]
             [small (stream-filter <pivot rest)]
             [large (stream-filter >=pivot rest)])
        (stream-append (qs small)
                       (stream-cons pivot
                                    (qs large))))))