(defstruct base ()
  (:method identify (self) (put-line "base")))

(defstruct derived (base)
  (:method identify (self) (put-line "derived")))

(defstruct poly ()
  obj

  (:method deep-copy (self)
    (let ((c (copy self))) ;; make copy of s
      (upd c.obj copy)     ;; copy self's obj
      c)))                 ;; return c

;; Test

(let* ((b (new base))
       (d (new derived))
       (p (new poly obj d)))
  b.(identify) ;; prints base
  d.(identify) ;; prints derived

  (let ((c p.(deep-copy)))
    p.obj.(identify) ;; prints derived
    (prinl (eq p.obj c.obj)))) ;; prints nil: c.obj is not a ref to p.obj