anchor.diff:  
   
   (defclass event ()
  -  ((id
  +  ((canceled-p
  +    :initform nil
  +    :accessor canceled-p-of
  +    :type boolean)
  +   (id
       :reader id-of
       :type id-type)
      (owner
  
positive.code:     sce))
  
  
  (defmethod synchronize ((sim simulator) tm)
    "Contract: simulator time -> simulator
  
  
negative.code: 
     Example: (multiple-value-call #'evolve (evolving example-scenario))"
    (let ((ev (pop (events-of sce))))
      (cond ((null ev) (values nil nil))
  	  ((canceled-p-of ev) (evolving sce))
  	  (t (values (owner-of ev) ev)))))
  