anchor.diff:  
      Description: the default lock policy is to not lock at all.
                   Specialize this method to provide your own lock
  -                policy."
  -  (assert (not (busy-p-of p)) nil "Locking a locked port.")
  +                policy. A timed lock must return the event that will
  +                unlock the port."
  +  (assert (not (lock-of p)) nil "Locking a locked port.")
  +  ;; do nothing
     nil)
   
   
  
positive.code: 
  (defmethod access-port ((sim simulator) (out out-port) (obj object))
    "Contract: simulator out-port object -> (or in-port nil)
                                            (or unlock-event nil)
  
     Purpose: to retrieve the in-port connected to out.
  
  
negative.code: 
  
  (defmethod lock-port ((p port) (obj object))
    "Contract: port object -> (or event nil)
  
     Purpose: to lock the port and decide when unlock it.
  
  