1. Computer Science Department, Universidade Federal de Minas Gerais, Belo Horizonte, Brazil
2. LIX, École Polytechnique, Palaiseau, France
3. CNRS, École Polytechnique, Palaiseau, France
4. Dipartimento di Informatica, Università di Pisa, Pisa, Italy
5. INRIA, École Polytechnique, Palaiseau, France