def joint_value_post_processor(a, _):
    """The final step in calculating joint values like disability weights.
    If the combiner is list_combiner then the effective formula is:

    .. math::

        value(args) = 1 -  \prod_{i=1}^{mutator count} 1-mutator_{i}(args)

    Parameters
    ----------
    a : List[pd.Series]
        a is a list of series, indexed on the population. Each series
        corresponds to a different value in the pipeline and each row
        in a series contains a value that applies to a specific simulant.
    """
    # if there is only one value, return the value
    if len(a) == 1:
        return a[0]

    # if there are multiple values, calculate the joint value
    product = 1
    for v in a:
        new_value = (1-v)
        product = product * new_value
    joint_value = 1 - product
    return joint_value