code: def test_multiindex(self, df1):
      res = df1.melt()
      assert res.columns.tolist() == ["CAP", "low", "value"]
  
imports: import re
  import numpy
  import pytest
  import pandas
  from pandas import DataFrame, Index, date_range, lreshape, melt, wide_to_long
  import pandas._testing
label_desc: Use image node_id 13 for calling the TestMelt obj's underlying member method code with example usage: obj.test_multiindex(df1) without return types