code: def test_vars_work_with_multiindex(self, df1):
      expected = DataFrame(
          {
              ("A", "a"): df1[("A", "a")],
              "CAP": ["B"] * len(df1),
              "low": ["b"] * len(df1),
              "value": df1[("B", "b")],
          },
          columns=[("A", "a"), "CAP", "low", "value"],
      )
  
      result = df1.melt(id_vars=[("A", "a")], value_vars=[("B", "b")])
      tm.assert_frame_equal(result, expected)
  
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 6 for calling the TestMelt obj's underlying member method code with example usage: obj.test_vars_work_with_multiindex(df1) without return types