code: public Observable<Void> registerAsync(String userName) {
          return registerWithServiceResponseAsync(userName).map(new Func1<ServiceResponse<Void>, Void>() {
              @Override
              public Void call(ServiceResponse<Void> response) {
                  return response.body();
              }
          });
      }
docs: Register a user to a managed lab.
  
  @param userName The name of the user.
  @throws IllegalArgumentException thrown if parameters fail the validation
  @return the {@link ServiceResponse} object if successful.