def product_sum(lags, Vx, Vt, k1, k2, k3, Cx, Ct):
    """Product-Sum space-time model

    Separable space-time variogram model, based on a combination of 'sum' and
    'product' models. Both base models are based on separated marginal
    variograms for the space and time axis.

    Parameters
    ----------
    lags : tuple
        Tuple of the space (x) and time (t) lag given as tuple: (x, t) which
        will be used to calculate the dependent semivariance.
    Vx : skgstat.Variogram.fitted_model
        instance of the space marginal variogram with a fitted theoretical
        model sufficiently describing the marginal. If this model does not fit
        the experimental variogram, the space-time model fit will be poor as
        well.
    Vt : skgstat.Variogram.fitted_model
        instance of the time marginal variogram with a fitted theoretical
        model sufficiently describing the marginal. If this model does not fit
        the experimental variogram, the space-time model fit will be poor as
        well.
    k1 : float
        Fitting parameter. k1 has to be positive or zero and may not be larger
        than all marginal sill values.
    k2 : float
        Fitting paramter. k2 has to be positive or zero and may not be larger
        than all marginal sill values.
    k3 : float
        Fitting parameter. k3 has to be positive and may not be larger than
        all marginal sill values.
    Cx : float
        Marginal space sill.
    Ct : float
        Marignal time sill.

    Returns
    -------
    gamma : float
        The semi-variance modeled for the given lags.

    Notes
    -----
    This model implements the product-sum model as suggested by
    De Cesare et. al [15]_, [16]_:

    .. math::
        \\gamma_{ST}(h_s, h_t) = [k_1C_T(0) + k_2]*\\gamma_S(h_s) +
        [k_1C_s(0) + k_3]\\gamma_T(h_t) - k_1\\gamma_s(h_s) x \\gamma_T(h_t)

    References
    ----------
    .. [15] De Cesare, L., Myers, D. and Posa, D. (2001a), Product-sum
       covariance for space-time mdeling, Environmetrics 12, 11-23.
    .. [16] De Cesare, L., Myers, D., and Pose, D. (201b), FORTRAN 77 programs
       for space-time modeling, Computers & Geoscience 28, 205-212.

    """
    h, t = lags
    return (k2 + k1*Ct)*Vx(h) + (k3 + k1*Cx) * Vt(t) - k1 * Vx(h) * Vt(t)