code: private static ActivationFunction natural(ErrorFunction error, int k) {
          if (error == ErrorFunction.CROSS_ENTROPY) {
              if (k == 1) {
                  return ActivationFunction.LOGISTIC_SIGMOID;
              } else {
                  return ActivationFunction.SOFTMAX;
              }
          } else {
              return ActivationFunction.LOGISTIC_SIGMOID;
          }
          
      }
docs: Returns the activation function of output layer based on natural pairing.
  @param error the error function.
  @param k the number of output nodes.
  @return the activation function of output layer based on natural pairing