code: def test_diff(self):
      res = self.res
      x = res.model.exog
  
      def fun(params):
          return np.dot(x, params) - np.dot(x[:, 1:], params[1:])
  
      nl = NonlinearDeltaCov(fun, res.params, res.cov_params())
      # the following two use broadcasting
      assert_allclose(nl.predicted(), res.params[0], rtol=1e-12)
      assert_allclose(nl.se_vectorized(), res.bse[0], rtol=1e-12)
  
file_path: statsmodels/statsmodels/stats/tests/test_deltacov.py
imports: import numpy
  from numpy.testing import assert_allclose, assert_equal
  from statsmodels.regression.linear_model import OLS
  from statsmodels.stats._delta_method import NonlinearDeltaCov
label_desc: Use image node_id 4 for calling the TestDeltacovOLS obj's underlying member method code with example usage: obj.test_diff() and returns: unknown