def add_NA_indicator_variables(df, inplace=False):
    """
    Add indicator variables for each column to indicate missingness.
    """
    df_ = df if inplace else df.copy()
    for i, c in enumerate(df_.columns):
        x = df_[c].isna()
        if x.any():
            df_.insert(i + 1, '{}_NA'.format(c), x)
    return df_