anchor.diff:        sim
         (add-children (add-child sim (first ch))
   		    (rest ch))))
  +
  +
  +(defmethod remove-children ((sim simulator) (ch list))
  +  (if (null ch)
  +      sim
  +      (remove-children (remove-child sim (first ch))
  +		       (rest ch))))
   
   
   (defmethod children ((sim simulator))
  
positive.code: 
  (defmethod do-output ((sim simulator) (evs list) (out out-port)
  		      (obj object))
    (restart-case (transition sim evs out obj)
      ;; waiting for the destination's in-port to be free
      (wait-in (sim evs in obj)
        (setf (waiting-queue-of in)
  
negative.code: 	      ;; else evolve and return values
  	      (multiple-value-bind (new-children new-evs)
  		  (evolve evolving evs)
  		(values (add-children sim new-children)
  			new-evs)))))))
  