Developing an indicator with other indicators

[Joe 0]

Hey, I would like to make an indicator for the following formulas:

SMA - 0.2ATR, SMA + 0.2ATR

I can source the value for SMA with bt.indicators.MovingAverageSimple and for ATR at bt.indicators.atr.AverageTrueRange my question is how can I combine them to make a sub-indicator that accounts both of these values?

It appears that SMA calculates the values on next() here

    def next(self):
        self.line[0] = \
            math.fsum(self.data.get(size=self.p.period)) / self.p.period

But ATR calculates it here:

        self.lines.atr = self.p.movav(TR(self.data), period=self.p.period)

(I tried to backtrack what self.p.movav is supposed to be but that wasn't so obvious)

How do I go about making this integrated indicator?