Top exchange warns about EU regulating foreign firms after Brexit

European Union attempts to supervise American financial firms as a condition of access to the bloc's markets would be firmly rejected, a top U.S. securities exchange official said on Tuesday.Mark Hemsley, president of the European arm of U.S. exchange CBOE, said EU regulators were trying to map out future working relations with their UK counterparts after Britain leaves the bloc next March.Brussels has proposed joint supervision of foreign clearing houses that want access to EU customers - or el