def _categorical_stats(idadf, stat, columns):
    """
    Computes various stats from one or several categorical columns of the IdaDataFrame.
    This is not implemented.

    Parameters
    ----------
    idadf : IdaDataFrame
    stat : str
        Name of the statistic function to be computed.
    columns : str or list of str
        Name of columns belonging to the IdaDataFrame.

    Returns
    -------
        Tuple.
    """
    raise NotImplementedError('TODO')
