char *
top_insert( up, down, init_step_ratio, ratio )
        char * up
        char * down
        double init_step_ratio
        double ratio
    INIT:
        int up_num, down_num, sample_num;
        double *return_val = NULL;
    PPCODE:
        up_num = {length of arg 0}/sizeof(double)/3;
        down_num = {length of arg 1}/sizeof(double)/3;
        return_val = get_inter_line(
            up_num,   (double*)up,
            down_num, (double*)down,
            init_step_ratio, ratio, &sample_num );
        sample_num *= 3;
        {set return value SV to:
            copy of return_val w/ length sample_num*sizeof(double)}
        del_inter_line(return_val);