code: def test_FreeGroupElm_letter_form():
      assert (x**3).letter_form == (
          Symbol("x"),
          Symbol("x"),
          Symbol("x"),
      )
      assert (x**2 * z**-2 * x).letter_form == (
          Symbol("x"),
          Symbol("x"),
          -Symbol("z"),
          -Symbol("z"),
          Symbol("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 13 for calling a global function with example usage: test_FreeGroupElm_letter_form() without return types