code: def test_set_index_append(self, frame_of_index_cols, drop, keys):
      df = frame_of_index_cols
  
      keys = keys if isinstance(keys, list) else [keys]
      idx = MultiIndex.from_arrays(
          [df.index] + [df[x] for x in keys], names=[None] + keys
      )
      expected = df.drop(keys, axis=1) if drop else df.copy()
      expected.index = idx
  
      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 11 for calling the TestSetIndex obj's underlying member method code with example usage: obj.test_set_index_append(frame_of_index_cols, drop, keys) without return types