Prop Trading By Volume

I heard Barney Frank say over the weekend at Davos that he had talked to somebody in the UK government, perhaps Lord Mandelson, and they think the best way to get at regulating prop trading is through some sort of measure of volume. However, he didn't offer any more details than that. Hypothetically, if we know the volume of customer requests and also the volume of trades the bank makes, then the difference, excluding some trades to mitigate overexposure and risk, is prop trading.

Again, details...details...