def linear_annuity_mapping_fprime(underlying, alpha0, alpha1):
    """linear_annuity_mapping_fprime
    first derivative of linear annuity mapping function.
    See :py:func:`linear_annuity_mapping_func`.
    The function calculates following formula:

    .. math::
        \\alpha^{\prime}(S) := \\alpha_{0.}

    where
    :math:`S` is underlying,
    :math:`\\alpha_{0}` is alpha0.

    :param float underlying:
    :param float alpha0:
    :param float alpha1: not used.
    :return: value of first derivative of linear annuity mapping function.
    :rtype: float.
    """
    return alpha0