mult-in-column

(defn parse-int [s]
   (Integer. (re-find  #"\d+" s )))

(defn mult [x y]
  "Умножение в столбик двух чисел в виде строки. На выход: число в виде строки.
  Отрицательные числа пока не перемножаются"
  (let [x (map #(parse-int (str %)) x)
        y (map #(parse-int (str %)) (reverse y))
        leftover (atom 0)
        result (atom 0)
        summarize-it (atom [])
        multiple (fn [unit-of-y a] (+ (* unit-of-y (last a)) @leftover))]
    (loop [a x zero-digit []]
      (when-not (empty? a)
        (reset! leftover 0)
        (swap! summarize-it
          (fn [n] (conj n
            (reduce
              (fn [coll-sum unit-of-y]
                (reset! result (rem (multiple unit-of-y a) 10))
                (reset! leftover (quot (multiple unit-of-y a) 10))
                (if (= unit-of-y (last y))
                  (into (vec (reverse (into (into coll-sum [@result]) [@leftover]))) zero-digit)
                  (into coll-sum [@result])))
            [] y))))
            (recur (drop-last a) (conj zero-digit 0))))
            (str (reduce ; суммируем перемноженные числа
              (fn [n m] (+ n (parse-int (apply str m))))
              0
              @summarize-it))))

(mult "547" "83")
(mult "78547" "583")
(mult "1" "0")
(mult "1" "1")
(mult "1" "01")
(mult "0" "01")