def nd_cross_variogram(x1, y2, x2, y1):
    """
    Inner most calculation step of cross-variogram

    This function is used in the inner most loop of `neighbour_diff_squared`.

    Parameters
    ----------
    x, y : np.array

    Returns
    -------
    np.array

    """
    res =  (x1 - x2)*(y1 - y2)

    return res