code: func (sub *subState) startAckSub(nc *nats.Conn, cb nats.MsgHandler) error {
  	ackSub, err := nc.Subscribe(sub.AckInbox, cb)
  	if err != nil {
  		return err
  	}
  	sub.Lock()
  	// Should not occur, but if it was already set,
  	// unsubscribe old and replace.
  	sub.stopAckSub()
  	sub.ackSub = ackSub
  	sub.ackSub.SetPendingLimits(-1, -1)
  	sub.Unlock()
  	return nil
  }
docs: // Subscribes to the AckInbox subject in order to process subscription's acks
  // if not already done.
  // This function grabs and releases the sub's lock.