code: func (d *DiskStorer) Set(key, value string) error {
  	if !validKey(key) {
  		return errNoSession{}
  	}
  
  	filePath := path.Join(d.folderPath, key)
  
  	d.mut.Lock()
  	defer d.mut.Unlock()
  
  	return ioutil.WriteFile(filePath, []byte(value), 0600)
  }
docs: // Set saves the value string to the session pointed to by the session id key.