code: def test_fit_options(self):
      am = arch_model(self.zero_mean, mean="Constant", vol="Constant")
      res = am.fit(first_obs=100)
      res.forecast()
      res = am.fit(last_obs=900)
      res.forecast()
      res = am.fit(first_obs=100, last_obs=900)
      res.forecast()
      res.forecast(start=100)
      res.forecast(start=200)
      am = arch_model(
          self.zero_mean, mean="Constant", vol="Constant", hold_back=20
      )
      res = am.fit(first_obs=100)
      res.forecast()
  
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 11 for calling the TestForecasting obj's underlying member method code with example usage: obj.test_fit_options() without return types