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Welcome, Monsieur Ambassador? France's permanent representative to the United Nations, Luc de la Barre de Nanteuil, has been unofficially chosen by Foreign Minister Claude Cheysson to succeed Bernard Vernier-Pailliez as France's ambassador to Washington later this year.

This choice is expected to meet stiff resistance both among President Mitterrand's advisers and in the upper reaches of the Socialist Party, where Mr. Nanteuil's staunch conservatism and his reputed skepticism regarding the North-South dialogue has made him unpopular.

Those who support him argue that he will be ideologically in tune with the Reagan administration and would thus assure smooth sailing for French-American relations.m