(setf (get 'fido 'birth-year)     '((value  2004)))
(setf (get 'fido 'supers)         '(dog))
(setf (get 'dog 'barks)           '((value t)))
(setf (get 'dog 'supers)          '(mammal))
(setf (get 'mammal 'warm-blooded) '((value t)))
(setf (get 'mammal 'legs)         '((value 4)))
(setf (get 'mammal 'age)      '((if-needed agefn)))

(defun agefn (frame)
  (- (current-year) (getslot frame 'birth-year)))

> (getslot 'fido 'birth-year)    ; stored
2002

> (getslot 'fido 'barks)   ; inherited from dog
T

> (getslot 'fido 'legs)    ; inherited from mammal
4

> (getslot 'fido 'age)     ; inherited, if-needed
3

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