(def b {false 0
        true  1})
(defn stop [_ _ x] x)
(defn id [x] x)

(defn pow-helper2 [n f a]
  (letfn [(start [m t]
            (let [done? (b (> m 1))
                  again ([stop pow-helper2] done?)
                  doit  ([id f] done?)]
              (again (dec m) f (doit t))))]
    (start n a)))

(defn pow-helper1 [n f a]
  (let [batch 111
        done? (b (< batch n))
        again ([stop pow] done?)
        donow ([n batch] done?)]
    (again (- n batch) f (pow-helper2 donow f a))))

(defn pow [n f a]
  (let [batch 11111
        done? (b (< batch n))
        again ([stop pow] done?)
        donow ([n batch] done?)]
    (again (- n batch) f (pow-helper1 donow f a))))