Clojure: Binary Reflected Gray Codes

(defn towers [n]
    (cond
      (= n 1) (list 1)
       :else
         (apply conj (towers (dec n)) n (towers (dec n)))))

(defn gray [n]
     (cond
        (= n 1) (list [0] [1])
        :else
        (let [firsthalf  (map #(conj  % 0) (gray (dec n)) )
              secondhalf (map #(conj  % 1) (reverse (gray (dec n))))  ]
          (concat  firsthalf secondhalf))))

(defn rank [code]
      (cond
       (= code [0]) 0
       (= code [1]) 1
       :else
        (cond
         (= 0 (last code))  (rank (vec (drop-last code)))
           :else
           (dec (- (Math/pow 2 (count code)) (rank (vec (drop-last code))))))))