code: def test_FreeGroupElm_words():
      w = x**5 * y * x**2 * y**-4 * x
      assert w.subword(2, 6) == x**3 * y
      assert w.subword(3, 2) == F.identity
      assert w.subword(6, 10) == x**2 * y**-2
  
      assert (
          w.substituted_word(0, 7, y**-1) == y**-1 * x * y**-4 * x
      )
      assert (
          w.substituted_word(0, 7, y**2 * x)
          == y**2 * x**2 * y**-4 * x
      )
  
imports: from sympy.combinatorics.free_groups import free_group, FreeGroup
  from sympy.core import Symbol
  from sympy.testing.pytest import raises
  from sympy.core.numbers import oo
label_desc: Use image node_id 21 for calling a global function with example usage: test_FreeGroupElm_words() without return types