anchor.diff:  
   (in-package :bounce)
   
  -(defparameter +num-balls+ 2)
  +(defparameter +num-balls+ 30)
   
   (defparameter +canvas-width+ 800)
   (defparameter +canvas-height+ 600)
  
positive.code:                  (declare (ignore widget))
                   (dolist (ball balls) (update ball))
                   (gtk-widget-queue-draw canvas)
                   runningp))
          (g-signal-connect window "destroy" (lambda (widget)
                                               (declare (ignore widget))
  
negative.code: (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
  