def handle_ack ack
      raise ArgumentError unless ack.control?
      op_type, tags = ack.control_op
      @sub_tracker.update op_type, tags
    end
def handle_ack ack
      raise ArgumentError unless ack.control?
      op_type, tags = ack.control_op
      @sub_tracker.update op_type, tags
    end
Maintain subscription status. Must be called by the user (or subclass)
 of this class, most likely in the thread created by #start.