请帮我写一段MT4的BOLL缩口后又开口的报警指标代码，请每段代码后面标注注释

以下是一段MT4的BOLL缩口后又开口的报警指标代码：

#property indicator_chart_window
#property indicator_buffers 2
#property indicator_color1 LimeGreen
#property indicator_color2 Red

extern int boll_period = 20;
extern double boll_deviation = 2.0;
extern int ma_method = MODE_SMA;
extern int shift = 0;

double ma_buffer[];
double std_dev_buffer[];
double upper_band[];
double lower_band[];

int init()
{
    SetIndexBuffer(0, upper_band);
    SetIndexBuffer(1, lower_band);

    SetIndexStyle(0, DRAW_LINE);
    SetIndexStyle(1, DRAW_LINE);
    SetIndexLabel(0, "Upper Bollinger Band");
    SetIndexLabel(1, "Lower Bollinger Band");

    SetIndexDrawBegin(0, boll_period + shift);
    SetIndexDrawBegin(1, boll_period + shift);

    IndicatorDigits(Digits + 1);

    return(0);
}

int start()
{
    int limit = Bars - IndicatorCounted();
    if (limit > 0)
    {
        ArrayResize(ma_buffer, limit);
        ArrayResize(std_dev_buffer, limit);

        for (int i = 0; i < limit; i++)
        {
            ma_buffer[i] = iMA(NULL, 0, boll_period, 0, ma_method, PRICE_CLOSE, i + shift);
            std_dev_buffer[i] = boll_deviation * iStdDev(NULL, 0, boll_period, 0, ma_method, PRICE_CLOSE, i + shift);
            upper_band[i] = ma_buffer[i] + std_dev_buffer[i];
            lower_band[i] = ma_buffer[i] - std_dev_buffer[i];

            if (i > 0)
            {
                // Bollinger band is narrowing down
                if (std_dev_buffer[i] < std_dev_buffer[i - 1])
                {
                    Alert("Bollinger band is narrowing down");
                }
                // Bollinger band is opening up
                if (std_dev_buffer[i] > std_dev_buffer[i - 1])
                {
                    Alert("Bollinger band is opening up");
                }
            }
        }
    }

    return(0);
}

代码注释如下：

#property indicator_chart_window       // 指标将显示在图表窗口中
#property indicator_buffers 2           // 定义2个指标缓冲区
#property indicator_color1 LimeGreen    // 第1个指标缓冲区颜色为绿色
#property indicator_color2 Red          // 第2个指标缓冲区颜色为红色

// 定义输入参数
extern int boll_period = 20;            // Bollinger带周期
extern double boll_deviation = 2.0;     // Bollinger带偏差
extern int ma_method = MODE_SMA;        // 移动平均线计算方法
extern int shift = 0;                   // 移动平均线偏移量

// 定义指标缓冲区
double ma_buffer[];
double std_dev_buffer[];
double upper_band[];
double lower_band[];

// 初始化指标
int init()
{
    SetIndexBuffer(0, upper_band);
    SetIndexBuffer(1, lower_band);

    SetIndexStyle(0, DRAW_LINE);
    SetIndexStyle(1, DRAW_LINE);
    SetIndexLabel(0, "Upper Bollinger Band");
    SetIndexLabel(1, "Lower Bollinger Band");

    SetIndexDrawBegin(0, boll_period + shift);
    SetIndexDrawBegin(1, boll_period + shift);

    IndicatorDigits(Digits + 1);

    return(0);
}

// 计算指标
int start()
{
    int limit = Bars - IndicatorCounted();
    if (limit > 0)
    {
        ArrayResize(ma_buffer, limit);
        ArrayResize(std_dev_buffer, limit);

        for (int i = 0; i < limit; i++)
        {
            ma_buffer[i] = iMA(NULL, 0, boll_period, 0, ma_method, PRICE_CLOSE, i + shift);
            std_dev_buffer[i] = boll_deviation * iStdDev(NULL, 0, boll_period, 0, ma_method, PRICE_CLOSE, i + shift);
            upper_band[i] = ma_buffer[i] + std_dev_buffer[i];
            lower_band[i] = ma_buffer[i] - std_dev_buffer[i];

            if (i > 0)
            {
                // Bollinger带正在缩小
                if (std_dev_buffer[i] < std_dev_buffer[i - 1])
                {
                    Alert("Bollinger band is narrowing down");
                }
                // Bollinger带正在扩大
                if (std_dev_buffer[i] > std_dev_buffer[i - 1])
                {
                    Alert("Bollinger band is opening up");
                }
            }
        }
    }

    return(0);
}