docs: def MACVOL(self,days,rev=0):
      """ Comparing yesterday volume is high, low or equal.
          return ↑,↓ or -
          與前一天 days 日成交量移動平均比較
          rev = 0
            回傳 ↑,↓ or -
          rev = 1
            回傳 1,-1 or 0
      """
      yesterday = self.stock_vol[:]
      yesterday.pop()
      yes_MAVOL = float(sum(yesterday[-days:]) / days)
      today_MAVOL = self.MAVOL(days)
  
      return self.high_or_low(today_MAVOL, yes_MAVOL,rev)