docs: def _deriv_logaddexp(x1, x2):
      """The derivative of f(x, y) = log(e^x + e^y)"""
      y1 = np.exp(x1)
      y2 = np.exp(x2)
      df_dx1 = y1 / (y1 + y2)
      df_dx2 = y2 / (y1 + y2)
      return np.vstack([df_dx1, df_dx2]).T