code: def test_holdback_ar(self):
      y = self.ar2_garch
      mod = arch_model(y, mean="AR", lags=1, hold_back=1)
      res_holdback = mod.fit(disp="off")
      mod = arch_model(y, mean="AR", lags=1)
      res = mod.fit(disp="off")
      assert_allclose(
          res_holdback.params, res.params, rtol=1e-4, atol=1e-4
      )
  
imports: import datetime
  from itertools import product
  import numpy
  from numpy.random import RandomState
  from numpy.testing import assert_allclose
  import pandas
  from pandas.testing import assert_frame_equal
  import pytest
  from arch.data import sp500
  from arch.tests.univariate.test_variance_forecasting import preserved_state
  from arch.univariate import APARCH, ARX, EGARCH, FIGARCH, GARCH, HARCH, HARX, ConstantMean, ConstantVariance, EWMAVariance, MIDASHyperbolic, RiskMetrics2006, ZeroMean, arch_model
  from arch.univariate.mean import _ar_forecast, _ar_to_impulse
label_desc: Use image node_id 21 for calling the TestForecasting obj's underlying member method code with example usage: obj.test_holdback_ar() without return types