anchor.diff:  (defgeneric mass (object))
   (defgeneric update (object))
   (defgeneric draw (cr object))
  +(defgeneric collision (object-1 object2))
  +(defgeneric velocity (object))
  +(defgeneric overlapp (object-1 object-2))
   
   (defclass ball ()
     ((density
  
positive.code:         (g-signal-connect stop-button "clicked"
                            (lambda (widget)
                              (declare (ignore widget))
                              (setf runningp nil)))
          (g-signal-connect add-button "clicked"
                            (lambda (widget)
  
negative.code:                  (declare (ignore widget))
                   (dolist (ball balls) (update ball))
                   (gtk-widget-queue-draw canvas)
                   (do-collisions balls)
                   (let ((text (format nil "Total system energy: ~2$"
                                       (reduce #'+(mapcar #'energy balls)))))
                     (gtk-label-set-text energy-label text))
                   runningp))
          (g-signal-connect window "destroy" (lambda (widget)
                                               (declare (ignore widget))
  