code: @Override
      public void start(final BaseCallback<Credentials, AuthenticationException> callback) {
          signUpRequest.start(new BaseCallback<DatabaseUser, AuthenticationException>() {
              @Override
              public void onSuccess(final DatabaseUser user) {
                  authenticationRequest.start(callback);
              }
  
              @Override
              public void onFailure(AuthenticationException error) {
                  callback.onFailure(error);
              }
          });
      }
docs: Starts to execute create user request and then logs the user in.
  
  @param callback called on either success or failure.