def MAC(self,days,rev = 0):
    """ Comparing yesterday price is high, low or equal.
        return ↑,↓ or -
        與前一天 days 日收盤價移動平均比較
        rev = 0
          回傳 ↑,↓ or -
        rev = 1
          回傳 1,-1 or 0
    """
    yesterday = self.raw_data[:]
    yesterday.pop()
    yes_MA = float(sum(yesterday[-days:]) / days)
    today_MA = self.MA(days)

    return self.high_or_low(today_MA, yes_MA, rev)