anchor.diff:    one."))
   
   
  +(defmethod clone ((w world))
  +  (let ((new-world (make-instance 'world :id (id-of w))))
  +    (setf (events-of new-world) (events-of w))
  +    (setf (history-of new-world) (history-of w))
  +    new-world))
  +
  +
   (defmethod run ((w world) (ev event))
  -  (let* ((new-world (copy-structure w))
  +  (let* ((new-world (clone w))
   	 (this (pop (events-of new-world))))
       (push new-world (history-of new-world))
       (assert (eql this ev) nil
  
positive.code:     :documentation "Past instances of this world, newer first.")
  
     (events
      :initarg events
      :initform ()
      :accessor events-of
      :type list
      :documentation "World's events.")))
  
  
  (defmethod initialize-instance :after ((w world) &key)
    (push w (history-of w)))
  
  
  (defgeneric run (world event)
  
negative.code:     with the old instances of world that are kept in the history.")))
  
  
  (defclass world (identifiable)
    ((history
      :initform ()
  