code: func saveUser(storage Storage, user User) error {
  	// Save the private key and registration
  	userData := new(UserData)
  	var err error
  	userData.Key, err = savePrivateKey(user.key)
  	if err == nil {
  		userData.Reg, err = json.MarshalIndent(&user, "", "\t")
  	}
  	if err == nil {
  		err = storage.StoreUser(user.Email, userData)
  	}
  	return err
  }
docs: // saveUser persists a user's key and account registration
  // to the file system. It does NOT register the user via ACME
  // or prompt the user. You must also pass in the storage
  // wherein the user should be saved. It should be the storage
  // for the CA with which user has an account.