code: def test_preserve_refs(datetime_series):
      seq = datetime_series.iloc[[5, 10, 15]]
      seq.iloc[1] = np.nan
      assert not np.isnan(datetime_series.iloc[10])
  
imports: from datetime import timedelta
  import re
  import numpy
  import pytest
  from pandas.errors import IndexingError
  from pandas import NA, DataFrame, Index, IndexSlice, MultiIndex, NaT, Series, Timedelta, Timestamp, concat, date_range, isna, period_range, timedelta_range
  import pandas._testing
label_desc: Use image node_id 20 for calling a global function with example usage: test_preserve_refs(datetime_series) without return types