More flexible group-by

flexigroup.clj

(defn flexigroup
  "Like group-by, but allows arbitrary calculation of keys and values from 
   source coll, (for instance, to roll up distinct particular elements of a 
   vector by distinct other parts) and arbitrary combinations of old and new
   vectors (for instance, to perform an efficient online count).
   (flexigroup identity identity conj [] coll) == group-by
   (flexigroup identity identity (fn [a b] (inc a)) 0) == online count
   (flexigroup #(subvec % 0 1) #(subvec % 1) conj #{} coll) == all the 
   distinct rests of the vectors, grouped by the first element"
  [key-f val-f combine-f init-v coll]
  (persistent! 
    (reduce
      (fn [ret x]
        (let [k (key-f x)]
          (assoc! ret k (combine-f (get ret k init-v) (val-f x)))))
      (transient {}) coll)))