def corr_schema(pol):
    """
    Parameters
    ----------
    pol : Dataset

    Returns
    -------
    corr_schema : list of list
        correlation schema from the POLARIZATION table,
        `[[9, 10], [11, 12]]` for example
    """

    # Select the single row out
    corrs = pol.NUM_CORR.data[0]
    corr_types = pol.CORR_TYPE.data[0]

    if corrs == 4:
        return [[corr_types[0], corr_types[1]],
                [corr_types[2], corr_types[3]]]  # (2, 2) shape
    elif corrs == 2:
        return [corr_types[0], corr_types[1]]    # (2, ) shape
    elif corrs == 1:
        return [corr_types[0]]                   # (1, ) shape
    else:
        raise ValueError("corrs %d not in (1, 2, 4)" % corrs)