(1) Information Systems Group, Dept. of Computer and Information Sciences,
Norwegian University of Science and Technology, NO-7491 Trondheim, Norway.
(2) CARE Technologies S.A., Pda. Madrigueres, 44. 03700 Denia, Alicante, Spain
(3) Dep. Mat. e Eng., Universidade da Madeira, Campus da Penteada, 9000-390 Funchal, Portugal.