code: func (d *DiskStorer) Get(key string) (value string, err error) {
  	if !validKey(key) {
  		return "", errNoSession{}
  	}
  
  	filePath := path.Join(d.folderPath, key)
  
  	d.mut.RLock()
  	defer d.mut.RUnlock()
  
  	_, err = os.Stat(filePath)
  	if err != nil {
  		return "", errors.Wrapf(err, "unable to stat session file: %s", filePath)
  	}
  
  	contents, err := ioutil.ReadFile(filePath)
  	if err != nil {
  		return "", errors.Wrapf(err, "unable to read file: %s", filePath)
  	}
  
  	return string(contents), nil
  }
docs: // Get returns the value string saved in the session pointed to by the
  // session id key.