a.rkt

(define-syntax-rule
  (CGpipeline cg prev-value init-value body ...)
  (match cg
  [(CG id np pls)
   (define (send-value v)
     (place-channel-put (list-ref pls (add1 id)) v))
   (define prev-value
    (if (= id 0)
      init-value
      (place-channel-get (car pls))))
   (define result (begin body ...))
   (unless (= id (sub1 np)) (send-value result))
   result]))