How to backtest an add to composite indicator?

I have created an indicator which tells how many stocks (out of 1600) are below rsi 30 and how many are above rsi 70 with the use of addtocomposite indicator. Now how do I backtest this indicator ? I know how to backtest any afl or function however in this case I have to link a new afl with this indicator which I don't know

This will buy either the current stock or group of stocks when the composite is > 100.

AddToComposite( RSI()<30, "~RSIcomp", "V");
Graph0 = x =  Foreign("~RSIcomp", "V");
Buy = x > 100;