code: func splitSheetIntoPrefixAndSuffix(data string) (string, string, error) {
  	// Split the sheet at the end of its SheetData tag so that more rows can be added inside.
  	sheetParts := strings.Split(data, endSheetDataTag)
  	if len(sheetParts) != 2 {
  		return "", "", errors.New("unexpected Sheet XML: SheetData close tag not found")
  	}
  	return sheetParts[0], sheetParts[1], nil
  }
docs: // splitSheetIntoPrefixAndSuffix will split the provided XML sheet into a prefix and a suffix so that
  // more spreadsheet rows can be inserted in between.