code: def test_rolling_groupby_with_fixed_forward_specific(
      df, window_size, expected
  ):
      # GH 43267
      indexer = FixedForwardWindowIndexer(window_size=window_size)
      result = (
          df.groupby("a")["b"]
          .rolling(window=indexer, min_periods=1)
          .mean()
      )
      tm.assert_series_equal(result, expected)
  
imports: import numpy
  import pytest
  from pandas import DataFrame, MultiIndex, Series, concat, date_range
  import pandas._testing
  from pandas.api.indexers import BaseIndexer, FixedForwardWindowIndexer
  from pandas.core.indexers.objects import ExpandingIndexer, FixedWindowIndexer, VariableOffsetWindowIndexer
  from pandas.tseries.offsets import BusinessDay
label_desc: Use image node_id 15 for calling a global function with example usage: test_rolling_groupby_with_fixed_forward_specific(df, window_size, expected) without return types