At the end of last week, Donald Tusk, the E.U."s President, said that, if Britain wanted talks to begin a new trade agreement that would preserve its access to the huge European market, it would have to make concessions in a number of areas, including the settlement of Britain's financial obligations to the E.U.; the legal protections that would be afforded E.U. citizens living in the United Kingdom; and the future of the border between Northern Ireland, which is leaving in the E.U., and the Republican of Ireland, which isn't.