pe_problem_21_fragment1.clj

(defn amicable? [a]
  (if (@amicable-nos a) true
      (let [b (d a)
            db (d b)]
        (if (and (not= a b) (= a db))
          (do (swap! amicable-nos assoc b true) true)
          false))))

pe_problem_21_fragment2.clj

(def amicable-nos (atom {}))

pe_problem_21_fragment3.clj

(swap! an-atom f & args)

pe_problem_21_fragment4.clj

(apply f @atom & args)