anchor.diff:      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
   
  
positive.code: 
     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))))
  
negative.code: 
  
  (defclass event ()
    ((canceled-p
      :initform nil
      :accessor canceled-p-of
      :type boolean)
     (id
      :reader id-of
      :type id-type)
     (owner
  