(defn combination
  ([alphabet tail data limit] (combination alphabet tail data limit '()))
  ([alphabet tail data limit result]
   (cond
     (empty? tail) result
     (= (count data) limit) (cons data result)
     :else (cond->> (combination alphabet (rest tail) data limit result)
             (or (empty? data) (not= (first tail) (first data)))
             (combination alphabet alphabet (cons (first tail) data) limit)))))