docs: def autocovariance(self, num_autocov=16):
          """
          Compute the autocovariance function from the ARMA parameters
          over the integers range(num_autocov) using the spectral density
          and the inverse Fourier transform.
  
          Parameters
          ----------
          num_autocov : scalar(int), optional(default=16)
              The number of autocovariances to calculate
  
          """
          spect = self.spectral_density()[1]
          acov = np.fft.ifft(spect).real
  
          # num_autocov should be <= len(acov) / 2
          return acov[:num_autocov]