[Clojure] Tally unique items in sequence

gistfile1.clj

(defn tally
  "tally unique things in a sequence"
  [seq]
  (reduce (fn [ag i] (update ag i (fnil inc 0))) {} seq))

;; (tally [:a :a :a :b :b :c]) 
;; => {:a 3, :b 2, :c 1}