code: def test_holdback_first_obs(self):
      y = self.ar2_garch
      mod = arch_model(y, hold_back=20)
      res_holdback = mod.fit(disp="off")
      mod = arch_model(y)
      res_first_obs = mod.fit(disp="off", first_obs=20)
      assert_allclose(res_holdback.params, res_first_obs.params)
  
      with pytest.raises(ValueError):
          res_holdback.forecast(start=18)
  
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 19 for calling the TestForecasting obj's underlying member method code with example usage: obj.test_holdback_first_obs() without return types