as a note, the source is:

  (defn frequencies
  "Returns a map from distinct items in coll to the number of times they appear."
  {:added "1.2"}
  [coll]
  (persistent!
   (reduce (fn [counts x]
             (assoc! counts x (inc (get counts x 0))))
           (transient {}) coll)))

which I think is pretty much just Raphael's solution.