ohassidi

(defn fold-by ;;; similar to group-by
  ([kf ;;; Key function to group on
    op ;;; operation how to combine two elements under the same key
    coll] ;; collection of maps
   (persistent!
    (reduce
     (fn [ret x] ;; ret is a map from (kf x) to xs, similar to the result of group-by
       (let [k (kf x) ;; derive the key
             v (get ret k)] ;; get the value already associated with the key if it exists
         (assoc! ret