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L. ?. Cnrs, 75775 Paris Cedex 16, France E-mail address: miguel.couceiro@dauphine.fr (E. Lehtonen) Computer Science and Communications Research Unit, rue Richard Coudenhove-Kalergi, L-1359 Luxembourg, Luxembourg and Centro dé Algebra da Universidade de Lisboa, Avenida Professor Gama Pinto, pp.1649-1652