1. Istituto di Informatica e Telematica, CNR, Pisa, Italy. E-mail: stefano.bistarelli@iit.cnr.it
2. Dipartimento di Scienze, Universitá degli Studi “G. D'annunzio”, Pescara, Italy. E-mail: bista@sci.unich.it
3. Department of Computer Science, University College Cork, Ireland. E-mail: s.foley@cs.ucc.ie
4. Cork Constraint Computation Centre, Department of Computer Science, University College Cork, Ireland. E-mail: b.osullivan@cs.ucc.ie