code: func dataFromSliceOrFile(data []byte, file string) ([]byte, error) {
  	if len(data) > 0 {
  		return data, nil
  	}
  	if len(file) > 0 {
  		fileData, err := ioutil.ReadFile(file)
  		if err != nil {
  			return []byte{}, err
  		}
  		return fileData, nil
  	}
  	return nil, nil
  }
docs: // dataFromSliceOrFile returns data from the slice (if non-empty), or from the file,
  // or an error if an error occurred reading the file