code: func (s *StorageOverseer) Set(w http.ResponseWriter, r *http.Request, value string) error {
  	// Reuse the existing cookie ID if it exists
  	sessID, _ := s.options.getCookieValue(w, r)
  
  	if len(sessID) == 0 {
  		sessID = uuid.NewV4().String()
  	}
  
  	err := s.Storer.Set(sessID, value)
  	if err != nil {
  		return errors.Wrap(err, "unable to set session value")
  	}
  
  	w.(cookieWriter).SetCookie(s.options.makeCookie(sessID))
  
  	return nil
  }
docs: // Set looks in the cookie for the session ID and modifies the session with the new value.
  // If the session does not exist it creates a new one.