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