def sigma(ab_sig, bb_sig):
    """ Perform combining rule to get A+A sigma parameter.
        Output units are whatever those are of the input parameters.

        :param ab_sig: A+B sigma parameter
        :type ab_sig: float
        :param ab_sig: B+B sigma parameter
        :type ab_sig: float
        :rtype: float
    """

    if ab_sig is not None and bb_sig is not None:
        aa_sig = 2.0 * ab_sig - bb_sig
    else:
        aa_sig = None

    return aa_sig