code: func SetTxID(channelHeader *cb.ChannelHeader, signatureHeader *cb.SignatureHeader) error {
  	txid, err := ComputeTxID(
  		signatureHeader.Nonce,
  		signatureHeader.Creator,
  	)
  	if err != nil {
  		return err
  	}
  	channelHeader.TxId = txid
  	return nil
  }
docs: // SetTxID generates a transaction id based on the provided signature header
  // and sets the TxId field in the channel header