[amibroker] How do I get the price and indicator values in the custom backtester?

[ezbentley]

This should be a very simple question but I am new to the custom backtester.

I would like to use the price, current portfolio equity, and other indicator 
values to 
calculate the position size with the following code. 

if (Status("action") == actionPortfolio) {
        bo = GetBacktesterObject();
        bo.PreProcess();
        for (bar = 0; bar < BarCount; bar++) {
                CurrentEquity = bo.Equity;
                for (sig = bo.GetFirstSignal(bar); sig; sig = 
bo.GetNextSignal(bar)) {
                        sig.PosSize = /* What should I put here? */
                }
                bo.ProcessTradeSignals(bar);
        }
        bo.ListTrades();
}

For example, if I want to set position size to CurrentEquity * Ref(C, -1) / 
Ref(ATR(10), -1), 
apparently that syntax will give me an error because I cannot access C and 
ATR(10) in the 
usual manner. 

Can anyone help me with implementing CurrentEquity * Ref(C, -1) / Ref(ATR(10), 
-1) 
inside the custom backtester?

Thanks in advance,