Optionally throttled chan

(defn throttle-chan [input]
  (let [throttle-chan (chan)
        output-chan (chan)]
    (go
     (loop [throttle-msec nil]
       (let [t-chan (when (= :disabled throttle-msec)
                         (timeout throttle-msec))
             [val c] (alt! [input throttle-chan])]
         (if (= c throttle-chan)
           (recur val)
           (do
             (>! output-chan val)
             (when t-chan
               (<! t-chan))
             (recur throttle-msec))))))
    [output-chan throttle-chan]))