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LONDON, UNITED KINGDOM - JUNE 18: Prime Minister David Cameron (C) and Martin Schulz, President of the European Parliament (R) attend a commemoration service to mark the 200th Anniversary of the Battle of Waterloo, at St Paul's Cathedral on June 18, 2015 in London, England. (Photo by Anthony Devlin - WPA Pool/Getty Images)

LONDON, UNITED KINGDOM - JUNE 18: Prime Minister David Cameron (C) and Martin Schulz, President of the European Parliament (R) attend a commemoration service to mark the 200th Anniversary of the Battle of Waterloo, at St Paul's Cathedral on June 18, 2015 in London, England. (Photo by Anthony Devlin - WPA Pool/Getty Images)