anchor.diff:  			  for dep in deps
   			  when
   			    (eql (dependency-binding-target dep)
  -				   (weak-pointer-value binding))
  +				   (dependency-binding-target binding))
   			  do (error 'dependency-exists
   				    :cell cell
   				    :event event
  
positive.code: 	  (loop for arg in args
  	     collect (make-weak-pointer arg)))
      (loop for arg in args
         do (add-dependent arg 'changed cell))))
  
  (defun evaluate-formula (cell)
    (let ((args (loop for arg in (arguments cell)
  
negative.code:       (loop for binding in event-dependents
  	 when (eql (dependency-binding-target binding)
  		   dependent)
  	 do (setf (gethash (hash event) (dependents cell))
  		    (delete binding event-dependents))))))
  
  (defgeneric dependents-of-event (event cell)
  