gistfile1.txt

(define (grayscale-pict s)
  (define buf (pict->argb-pixels s))
  (define gray-buf
    (apply bytes
           (append*
            (for/list ([i (in-range 0 (bytes-length buf) 4)])
              (define a (bytes-ref buf i))
              (define r (bytes-ref buf (+ i 1)))
              (define g (bytes-ref buf (+ i 2)))
              (define b (bytes-ref buf (+ i 3)))
              (define av (exact-floor (/ (+ r g b) 3)))
              (list a av av av)))))
  (argb-pixels->pict gray-buf (pict-width s)))