Happy numbers kata

happy.clj

(require ['clojure.set])
(def happy-numbers (set [1, 7, 10, 13, 19, 23, 28, 31, 32, 44, 49, 68, 70, 79, 82, 86, 91, 94, 97, 100]))
(def not-happy-numbers (clojure.set/difference (set (range 1 100)) happy-numbers))

(defn decompose-in-digits [n]
  (map #(Character/digit % 10) (str n)))

(def square #(*' % %))

(defn- happy? [n visited]
    (if (contains? visited n)
      false
      (if (= n 1)
        true
        (let [ndigits (decompose-in-digits n)
              sum-of-squares (apply + (map square ndigits))]
          (happy? sum-of-squares (conj visited n))))))

(defn is-happy-number [n]
  (happy? n #{}))


(defn is-happy-number-tail [a]
  (loop [n a
         visited #{}]
    (if (contains? visited n)
      false
      (if (= n 1)
        true
        (let [ndigits (decompose-in-digits n)
              sum-of-squares (apply + (map square ndigits))]
          (recur sum-of-squares (conj visited n)))))))

(require ['clojure.test :refer :all])

(deftest check-happy
  (doseq [x happy-numbers]
    (is (is-happy-number x))))

(deftest check-not-happy
  (doseq [x not-happy-numbers]
    (is (not (is-happy-number x)))))

(run-tests)

On line 8, why

(def square #(*' % %))

what is the star with the quote (*') operator? Different from * ? Lazy?

prevent overflow