def exclude_bck(df):
    """
    Exclude background/target sentences that don't have any domain labels.
    """
    print(f"Before excl: {len(df)=}")
    crit1 = "(background_sent or target_sent)"
    crit2 = "labels.astype('str') == '[0, 0, 0, 0, 0, 0, 0, 0, 0]'"
    to_excl = df.query(f"{crit1} and {crit2}")
    df = df.loc[~df.index.isin(to_excl.index)]
    print(f"After excl: {len(df)=}")
    return df