;;assert(succ("") == "") ;;assert(succ("R2D3") == "R2D4") ;;assert(succ("R2D9") == "R2E0") ;;assert(succ("A99") == "B00") ;;assert(succ("Z99") == "AA00") ;;assert(succ("Zz99") == "AAa00") ;;assert(succ("9Z") == "10A")

gistfile1.clj

(defn  increment
  [str-chars]
  (if ((complement empty?) str-chars)
  (let [last-char (last str-chars)
          ahead-chars (drop-last str-chars)]
    (cond
     (=  last-char  \9) (if (empty? ahead-chars) "10" (str (increment  ahead-chars) \0))
     (= last-char \z) (if (empty? ahead-chars) "aa" (str (increment ahead-chars) \a))
     (= last-char \Z)  (if (empty? ahead-chars) "AA" (str (increment ahead-chars) \A))
     :else (apply str (conj  (vec ahead-chars) (char (inc (int last-char))))))) ""))
;;assert(succ("") == "")
;;assert(succ("R2D3") == "R2D4")
;;assert(succ("R2D9") == "R2E0")
;;assert(succ("A99") == "B00")
;;assert(succ("Z99") == "AA00")
;;assert(succ("Zz99") == "AAa00")
;;assert(succ("9Z") == "10A")

(increment (char-array "9"))
(increment (char-array "Z"))

(increment (char-array ""))

(increment (char-array "R2D3"))

(increment (char-array "R2D9"))

(increment (char-array "A99"))

(increment (char-array "Z99"))

(increment (char-array "Zz99"))

(increment (char-array "9Z"))