code: def test_design_expression():
      t1 = F.Term("x")
      t2 = F.Term("y")
      f = t1.formula + t2.formula
      assert str(f.design_expr) in ["[x, y]", "[y, x]"]
  
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 7 for calling a global function with example usage: test_design_expression() without return types