code: def _deriv_logaddexp2(x1, x2):
      
      y1 = np.exp2(x1)
      y2 = np.exp2(x2)
      df_dx1 = y1 / (y1 + y2)
      df_dx2 = y2 / (y1 + y2)
      return np.vstack([df_dx1, df_dx2]).T
docs: The derivative of f(x, y) = log2(2^x + 2^y)