(defstruct animal nil
  name
  (:method get-name (me)
    (if me.name me.name (error `get-name: animal @me has no name`)))
  (:method speak (me stream)
    (error "abstract animal cannot speak")))

(defstruct dog animal
  (:method speak (me : (stream *stdout*))
    (put-line `@{me.(get-name)}: bark!` stream)))

(defstruct cat animal
  (:method speak (me : (stream *stdout*))
    (put-line `@{me.(get-name)}: meow!` stream)))

(defstruct lab dog)

(defstruct collie dog)

(let ((pet1 (new collie name "Lassie"))
      (pet2 (new cat name "Max")))
  pet1.(speak)
  pet2.(speak))