code: def test_cv_exceptions():
      with pytest.raises(ValueError, match="test_type must be one of"):
          phillips_ouliaris_cv("unknown", "c", 2, 500)
      with pytest.raises(ValueError, match="trend must by one of:"):
          phillips_ouliaris_cv("Pu", "unknown", 2, 500)
      with pytest.raises(
          ValueError, match="The number of stochastic trends must"
      ):
          phillips_ouliaris_cv("Pu", "ct", 25, 500)
      with pytest.warns(CriticalValueWarning):
          phillips_ouliaris_cv("Pu", "ct", 2, 10)
  
file_path: arch/arch/tests/unitroot/test_phillips_ouliaris.py
imports: import numpy
  from numpy.testing import assert_allclose
  import pytest
  from statsmodels.iolib.summary import Summary
  from arch.typing import Float64Array, UnitRootTrend
  from arch.unitroot._phillips_ouliaris import CriticalValueWarning, phillips_ouliaris_cv, phillips_ouliaris_pval
  from arch.unitroot.cointegration import phillips_ouliaris
  from arch.utility.timeseries import add_trend
label_desc: Use image node_id 7 for calling a global function with example usage: test_cv_exceptions() without return types