anchor.diff:                  unlock.
                   Specialize this method to provide your own lock
                   policy."
  -  (setf (lock-of p) t)
     (list (make-instance 'event
   		       :owner sim
   		       :time (clock-of sim)
  
positive.code: 
  
  (defmethod lock-port :before ((sim simulator) (p port) (obj object))
    (assert (not (lock-of p)) nil "Locking a locked port."))
  
  
  (defmethod lock-port ((sim simulator) (p port) (obj object))
  
negative.code: 
  (defmethod unlock-port ((sim simulator) (out out-port))
    "Contract: out-port -> events"
    (list (make-instance 'event
  		       :owner sim
  		       :time (clock-of sim)
  