数字を各桁に分解(10進数限定)

How do you extract digits from an integer in java? - Yahoo! Answers を見て
なるほどど思い早速 Clojure で書いてみる。

(defn digits [n]
  (letfn [(digits-internal [n]
            (when-not (zero? n)
              (cons (mod n 10) (digits-internal (quot n 10)))))]
    (reverse (digits-internal n))))