code: func Exists(path string) (bool, error) {
  	path, err := filepath.Abs(filepath.Join(path, keysBoltFile))
  	if err != nil {
  		return false, trace.Wrap(err)
  	}
  	f, err := os.Open(path)
  	err = trace.ConvertSystemError(err)
  	if err != nil {
  		if trace.IsNotFound(err) {
  			return false, nil
  		}
  		return false, trace.Wrap(err)
  	}
  	defer f.Close()
  	return true, nil
  }
docs: // Exists returns true if backend has been used before