code: def add_subscriber(self, subscriber, precedence=0):
          
          if not callable(subscriber):
              raise TypeError('Subscriber must be a callable.')
          self._subscribers.append((precedence, subscriber))
docs: Register a callable subscriber to this stream which will be
          invoked either when event is called or when this stream is
          passed to the trigger classmethod.
  
          Precedence allows the subscriber ordering to be
          controlled. Users should only add subscribers with precedence
          between zero and one while HoloViews itself reserves the use of
          higher precedence values. Subscribers with high precedence are
          invoked later than ones with low precedence.