code: def test_rolling_cov(window, center):
      # DataFrame
      prolling = df.drop("a", axis=1).rolling(window, center=center)
      drolling = ddf.drop("a", axis=1).rolling(window, center=center)
      assert_eq(prolling.cov(), drolling.cov())
  
      # Series
      prolling = df.b.rolling(window, center=center)
      drolling = ddf.b.rolling(window, center=center)
      assert_eq(prolling.cov(), drolling.cov())
  
imports: from __future__ import annotations
  import contextlib
  import datetime
  import numpy
  import pandas
  import pytest
  import dask.dataframe
  from dask.dataframe._compat import PANDAS_GE_210
  from dask.dataframe.utils import assert_eq
label_desc: Use image node_id 9 for calling a global function with example usage: test_rolling_cov(window, center) without return types