def get_varname_Nd(fromNd, toNd):
    """
    returns variable name of N_x<dy.
    :param fromNd:
    :param toNd:
    :return:
    """
    if fromNd > 0:
        varNameN = 'N%d_d_%d' % (fromNd, toNd)
    else:
        varNameN = 'Nd_%d' % toNd
    return varNameN