code: func getSheetIndex(sf *StreamFile, path string) (int, error) {
  	indexString := path[len(sheetFilePathPrefix) : len(path)-len(sheetFilePathSuffix)]
  	sheetXLSXIndex, err := strconv.Atoi(indexString)
  	if err != nil {
  		return -1, errors.New("Unexpected sheet file name from xlsx package")
  	}
  	if sheetXLSXIndex < 1 || len(sf.sheetXmlPrefix) < sheetXLSXIndex ||
  		len(sf.sheetXmlSuffix) < sheetXLSXIndex || len(sf.xlsxFile.Sheets) < sheetXLSXIndex {
  		return -1, errors.New("Unexpected sheet index")
  	}
  	sheetArrayIndex := sheetXLSXIndex - 1
  	return sheetArrayIndex, nil
  }
docs: // getSheetIndex parses the path to the XLSX sheet data and returns the index
  // The files that store the data for each sheet must have the format:
  // xl/worksheets/sheet123.xml
  // where 123 is the index of the sheet. This file path format is part of the XLSX file standard.