def seq_idx(df):
    """
    Provide boolean indexer for South East Queensland

    Parameters:
    -------
    df : dataframe
        a population dataframe

    Raises
    ------
    ValueError
        if the dataframe does not contain an asgs_name column

    Returns
    -------
    idx_seq : boolean indexer   
        an indexer to Tudge's definition of SEQ.
    """

    if "asgs_name" not in df.columns:
        raise ValueError("asgs_name column not in dataframe")

    idx_seq = (
        (df.asgs_name == "Greater Brisbane")
        | (df.asgs_name == "Sunshine Coast")
        | (df.asgs_name == "Gold Coast")
        | (df.asgs_name == "Toowoomba")
    )

    return idx_seq