Even the whiff of a country’s likely exit from eurozone membership could cause a run on that country’s banks and become a self-fulfilling prophecy. That is the logical conclusion of an exercise that few within the eurozone, or even outside it, dare to rehearse. It could destroy the euroland banking system. But the European Commission’s own president, Romano Prodi, has twice raised the taboo subject of a euro exit. The intellectual challenge of predicting how things would work out won’t go away. Brian Kettell takes us through a hypothetical French exit.