Theresa May, the British prime minister, recently sent a letter to Donald Tusk, president of the European Council, announcing that the UK would withdraw from the EU under the procedures set out in Article 50 of the Lisbon Treaty. Her letter noted that, even though withdrawal was irrevocable, the UK seeks to forge the closest and most cooperative arrangement possible with the EU moving forward. Although Article 50 prevents the EU from blocking Brexit, it offers little by way of guidance on how the exit negotiations should proceed.