code: func (ed *Dispatcher) updateLastBlockNum(blockNum uint64) error {
  	// The Deliver Service shouldn't be sending blocks out of order.
  	// Log an error if we detect this happening.
  	lastBlockNum := atomic.LoadUint64(&ed.lastBlockNum)
  	if lastBlockNum == math.MaxUint64 || blockNum > lastBlockNum {
  		atomic.StoreUint64(&ed.lastBlockNum, blockNum)
  		logger.Debugf("Updated last block received to %d", blockNum)
  		return nil
  	}
  	return errors.Errorf("Expecting a block number greater than %d but received block number %d", lastBlockNum, blockNum)
  }
docs: // updateLastBlockNum updates the value of lastBlockNum and
  // returns the updated value.