TRADERS OF OIL futures are a sunny bunch. On April 17th, after Iran’s foreign minister declared the Strait of Hormuz “completely open”, the price of Brent crude fell by 10%, to $90 a barrel. Within hours Iran reversed course and attacked an Indian tanker. The next trading day the global benchmark rose by just 5%. It remains around $20 below its high in late March, even though an American blockade on Iranian oil means even more oil is trapped in the Gulf.