(defstruct shape ()
  cached-area

  (:init (self)
    (put-line `@self is born!`))

  (:fini (self)
    (put-line `@self says goodbye!`))

  (:method area (self)
    (or self.cached-area
        (set self.cached-area self.(calc-area)))))

(defstruct circle shape
  (radius 1.0)

  (:method calc-area (self)
    (* %pi% self.radius self.radius)))

(defstruct square shape
  (length 1.0)

  (:method calc-area (self)
    (* self.length self.length)))