anchor.diff:      :documentation "Past instances of this world, newer first.")
   
      (events
  -    :initarg events
  -    :initform ()
  +    :initarg :events
  +    :initform (error "missing :events")
       :accessor events-of
       :type list
       :documentation "World's events.")))
   
   
  -(defmethod initialize-instance :after ((w world) &key)
  -  (push w (history-of w)))
  +(defgeneric clone (object)
  +  (:documentation "Return a shallow copy of object."))
   
   
   (defgeneric run (world event)
  
positive.code: (defpackage :org.altervista.rjack.desim
    (:nicknames :ds)
    (:use :common-lisp)
    (:export :event
  	   :object
  	   :world
  	   :run))
  
  (in-package :ds)
  
  
negative.code:     with the old instances of world that are kept in the history.")))
  
  
  (defclass world (identifiable)
    ((history
      :initform ()
  