Like leg warmers and mullets, the Volcker rule sounded like a great idea to someone at the time. It couldn’t have found a better namesake than former Fed chief Paul Volcker, who called loudly for a prohibition on proprietary trading by US banks - in-house operations that trade everything from stocks and bonds to gold, oil, and futures. Lately, however, the rule has come under heavy fire. Not by angry bankers, but by government regulators and central-bank officials around the world.