code: def test_casefold():
      # GH25405
      expected = Series(["ss", np.nan, "case", "ssd"])
      s = Series(["ß", np.nan, "case", "ßd"])
      result = s.str.casefold()
  
      tm.assert_series_equal(result, expected)
  
imports: from datetime import datetime
  import operator
  import numpy
  import pytest
  from pandas import Series, _testing
label_desc: Use image node_id 9 for calling a global function with example usage: test_casefold() without return types