Whatever else you might say about investment banks, please don’t mention the “p” word. Not “p” for pay, but “p” for proprietary trading.

As the debate over the future of the "p" word rumbles on, you might have thought that the new, low-fat, slimline Wall Street had already moved on. Investment banks have been falling over themselves over the past few years to explain that they don't do the "p" word any more. But if that's true, why do they still get so worked up about it?