European Council President Donald Tusk arrives at the Government office ahead of the upcoming European Union summit- the first one since Britain voted to quit- in Bratislava, Slovakia, September 15, 2016. REUTERS/Yves Herman

European Council President Donald Tusk (3rdL), European Commission President Jean-Claude Juncker (2ndR) and European Parliament President Martin Schulz (L) pose for a group photo with Slovakia's Prime Minister Robert Fico (C), Slovakia's President Andrej Kiska (3rdR) and Speaker of the Slovakia's National Council Andrej Danko (2ndL) at the Government office ahead of the upcoming European Union summit- the first one since Britain voted to quit- in Bratislava, Slovakia, September 15, 2016. REUTERS/Yves Herman

Sweden's Rikard Karlberg on tee three during the last round at Bro Hof golf club during the Nordea Masters tournament in Stockholm, Sweden June 5, 2016. TT News Agency/Fredrik Persson/via REUTERS

The M1 3D printer prints a sophisticated geometric structure, developed by Silicon Valley startup Carbon, at Carbon headquarters in Redwood City, California, U.S., September 9, 2016. Picture taken September 9, 2016. REUTERS/Ben Gruber

Traders work on the floor of the New York Stock Exchange (NYSE) in New York City, U.S., September 15, 2016. REUTERS/Brendan McDermid

Traders work on the floor of the New York Stock Exchange (NYSE) in New York City, U.S., September 15, 2016. REUTERS/Brendan McDermid

Traders react while working on the floor of the New York Stock Exchange (NYSE) in New York City, U.S., September 15, 2016. REUTERS/Brendan McDermid

People sit outside the New York Stock Exchange (NYSE) in New York City, U.S., September 15, 2016. REUTERS/Brendan McDermid