code: def add_subscriber(self, connection_id, subscriptions,
                         last_known_block_id):
          
          with self._subscribers_cv:
              self._subscribers[connection_id] = \
                  EventSubscriber(
                      connection_id, subscriptions, last_known_block_id)
  
          LOGGER.debug(
              'Added Subscriber %s for %s', connection_id, subscriptions)
docs: Register the subscriber for the given event subscriptions.
  
          Raises:
              InvalidFilterError
                  One of the filters in the subscriptions is invalid.