Multiset - Clojure

(defn add [multiset elem] (assoc multiset elem (+ (get multiset elem 0) 1)))
(defn remove [multiset elem] (case (get multiset elem 0)
                               0 multiset
                               1 (dissoc multiset elem 1)
                               (assoc multiset elem (- (get multiset elem) 1))))
(defn union [multiset other] (reduce
                               (fn [m k]
                                 (assoc m k (+ (get m k 0) (get other k))))
                               multiset (keys other)))
(defn intersect [multiset other] (reduce
                                   (fn [m k] (if (contains? multiset k) (assoc m k (min (get multiset k) (get other k)))))
                                   {} (keys other)))
(defn toString [multiset] (str "Bag[" (subs (reduce
                                              (fn [s k] (str s ", " k ":" (get multiset k)))
                                              "" (keys multiset)) 2) "]"))