U.K. Prime Minister Theresa May Attends European Union Leaders Summit [Getty Images]

Paolo Gentiloni, Italy's prime minister, speaks during a news conference after a meeting of European Union (EU) leaders in Brussels, Belgium, on Thursday, Dec. 15, 2016. A first glimpse of the European Union's potential for disunity on Brexit emerged at the Brussels summit, as EU leaders were caught up in a dispute over how the bloc negotiates with the U.K. as it heads for the door. Photographer: Jasper Juinen/Bloomberg via Getty Images

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