Eager cartesian product between two seqs in Clojure

prod.clj

(defn- pair
  [k v]
  (clojure.lang.MapEntry. k v))

(defn prod
  [xs ys]
  (persistent!
   (reduce
    (fn [acc x]
      (reduce
       (fn [acc' y]
         (conj! acc' (pair x y)))
       acc
       ys))
    (transient [])
    xs)))