code: def test_empty(self):
      assert_equal(ros._detection_limit_index(None, self.empty_cohn), 0)
  
imports: from statsmodels.compat.pandas import assert_series_equal, assert_frame_equal
  from io import StringIO
  from textwrap import dedent
  import numpy
  import numpy.testing
  import numpy
  from numpy.testing import assert_equal
  import pandas
  import pytest
  from statsmodels.imputation import ros
label_desc: Use image node_id 2 for calling the Test__detection_limit_index obj's underlying member method code with example usage: obj.test_empty() without return types