What is Goldman Sachs up to? When Michael Lewis unveiled his book Flash Boys, the bank discreetly lent him support. Then it emerged that Goldman is leaving the New York Stock Exchange floor, selling Spear, Leeds & Kellogg, a broker it bought for $6.5bn in 2000.

Are we witnessing a classic Wall Street boom and bust cycle where banks rush into a business that is profitable for those with an edge, but others eventually catch up and margins are eroded.