anchor.diff:    (:use :common-lisp)
     (:export :!transform :clone :run :next :prev
   	   :identifiable :id-of
  -	   :simulated :description-of :effects-of
  -	   :event :time-of :causes-of :!action-of
  +	   :simulated :description-of
  +	   :event :time-of :!action-of
   	   :world :hystory-of :events-of))
   
   
  
positive.code:     :reader time-of
      :type fixnum
      :documentation "When this event must be executed.")
  
     (causes
      :initarg :causes
      :initform (error "missing :causes")
      :accessor causes-of
      :type list
      :documentation "List of objects that will cause this event to
      happen.")
  
     (!action
      :initarg :!action
  
negative.code:     :initform "simulated object"
      :accessor description-of
      :type string
      :documentation "Description of this object.")))
  
  
  (defclass event (identifiable)
  