(defclass object () ())
(defclass ship (object) ())
(defclass asteroid (object) ())
 
(defgeneric collide (a b))
 
(defmethod collide ((s ship) (o object))
  (format t "Ship hits something: ~a" o))
 
(defmethod collide ((o object) (a asteroid))
  (format t "Something ~a hits asteroid" o))
 
(defmethod collide ((a object) (b object))
  (format t "Something ~a hits something ~a" a b))
 
(collide (make-instance 'ship) (make-instance 'asteroid))

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