如何在Clojure中扩展数值协议

(defprotocol INumber
  (add [x y])
  (subtract [x y])
  (multiply [x y])
  (divide [x y])
  (remainder [x y])
  (negate [x]))

(extend-protocol INumber
  Money
  (add [x y] (Money. (+ (:amount x) (:amount y))))
  (subtract [x y] (Money. (- (:amount x) (:amount y))))
  (multiply [x y] (Money. (* (:amount x) (:amount y))))
  (divide [x y] (Money. (/ (:amount x) (:amount y))))
  (remainder [x y] (Money. (rem (:amount x) (:amount y))))
  (negate [x] (Money. (- (:amount x)))))

(def m1 (Money. 10))
(def m2 (Money. 20))

(+ m1 m2) ;=> #Money{:amount 30}
(- m1 m2) ;=> #Money{:amount -10}
(* m1 m2) ;=> #Money{:amount 200}
(/ m1 m2) ;=> #Money{:amount 0.5}
(% m1 m2) ;=> #Money{:amount 10}
(- m1)   ;=> #Money{:amount -10}