(def latest (ref nil))
(def changelists (channel))

(receive-all changelists
  (fn [changelist]
    (dosync
      (ref-set latest changelist))))

(defn request-handler [ch request]
  (let [last-serial (-> request :body :last-serial)
	src (dosync
	      (if (and @latest (> (:serial latest) last-serial))
		(sealed-channel @latest)
		(take* 1 changelists)))]
    (siphon (map* create-http-response src) ch)))