There is, perhaps, one iron law of human nature. Whenever a claim is obsessively and strenuously denied at every possible opportunity, the allegation in question is almost certainly true. So it is that the diplomats of the Islamic Republic of Iran – a caste of black-clad men who are mordant, fluent, adamant and able – invariably deny that settling the nuclear question had anything to do with the sanctions imposed on their country’s economy.Here in Vienna, where the agreement has just been signed, Mohammad Javad Zarif, the accomplished and incisive Iranian Foreign Minister, insists that his government’s desire for “constructive engagement” with the world – and nothing else – lay behind the deal.Back at home, however, the message is rather dif