code: def test_set_index_append_to_multiindex(
      self, frame_of_index_cols, drop, keys
  ):
      # append to existing multiindex
      df = frame_of_index_cols.set_index(["D"], drop=drop, append=True)
  
      keys = keys if isinstance(keys, list) else [keys]
      expected = frame_of_index_cols.set_index(
          ["D"] + keys, drop=drop, append=True
      )
  
      result = df.set_index(keys, drop=drop, append=True)
  
      tm.assert_frame_equal(result, expected)
  
imports: from datetime import datetime, timedelta
  import numpy
  import pytest
  from pandas import Categorical, CategoricalIndex, DataFrame, DatetimeIndex, Index, MultiIndex, Series, date_range, period_range, to_datetime
  import pandas._testing
label_desc: Use image node_id 12 for calling the TestSetIndex obj's underlying member method code with example usage: obj.test_set_index_append_to_multiindex(frame_of_index_cols, drop, keys) without return types