anchor.diff:  
      Example: (multiple-value-call #'evolve (evolving example-scenario))"
     (let ((ev (pop (events-of sce))))
  -    (if (null ev)
  -	(values nil nil)
  -	(values (owner-of ev) ev))))
  +    (cond ((null ev) (values nil nil))
  +	  ((canceled-p-of ev) (evolving sce))
  +	  (t (values (owner-of ev) ev)))))
  
positive.code: 
  
  (defclass event ()
    ((id
      :reader id-of
      :type id-type)
     (owner
  
negative.code:     sce))
  
  
  (defmethod cancel-event ((ev event))
    "Contract: event -> nil
  
     Purpose: to set the canceled flag of ev by side effect."
    (setf (canceled-p-of ev) t)
    nil)
  
  
  (defmethod synchronize ((sim simulator) tm)
    "Contract: simulator time -> simulator
  
  