1. a IBM Research, T. J. Watson Research Center , P.O. Box 218, Yorktown Heights , New York , 10598 , U.S.A.
2. b Saint-Gobain Recherche , BP 135, 93303 Aubervilliers Cedex, France
3. c Thomson-CSF, Laboratoire Central de Recherche , Domaine de Corbeville, BP 10, 91401 , Orsay , France