code: def hessian(self, x, y, kwargs, diff=diff):
          
          alpha_ra, alpha_dec = self.alpha(x, y, kwargs)
  
          alpha_ra_dx, alpha_dec_dx = self.alpha(x + diff, y, kwargs)
          alpha_ra_dy, alpha_dec_dy = self.alpha(x, y + diff, kwargs)
  
          dalpha_rara = (alpha_ra_dx - alpha_ra)/diff
          dalpha_radec = (alpha_ra_dy - alpha_ra)/diff
          dalpha_decra = (alpha_dec_dx - alpha_dec)/diff
          dalpha_decdec = (alpha_dec_dy - alpha_dec)/diff
  
          f_xx = dalpha_rara
          f_yy = dalpha_decdec
          f_xy = dalpha_radec
          f_yx = dalpha_decra
          return f_xx, f_xy, f_yx, f_yy
docs: computes the differentials f_xx, f_yy, f_xy from f_x and f_y
          :return: f_xx, f_xy, f_yx, f_yy