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