code: def test_stratify():
      fac = F.Factor("x", [2, 3])
  
      y = sympy.Symbol("y")
      f = sympy.Function("f")
      pytest.raises(ValueError, fac.stratify, f(y))
  
imports: from warnings import catch_warnings, simplefilter
  import numpy
  import pytest
  import sympy
  from numpy.core.records import fromrecords
  from numpy.testing import assert_almost_equal, assert_array_equal
  from sympy.utilities.lambdify import implemented_function
  from nipy.utils import VisibleDeprecationWarning
  from ..None import formulae
  from ..formulae import Term, terms
label_desc: Use image node_id 21 for calling a global function with example usage: test_stratify() without return types