Acknowledgments

The authors wish to thank Manuel Palomar for his helpful revisions of this paper; Ferr�n Pla, Ruslan Mitkov, and Richard Evans for having contributed their corpora; and Rafael Muñoz, Maximiliano Saiz-Noeda, Patricio Martínez-Barco, and Juan Carlos Trujillo for their suggestions and willingness to help in any task related to this paper. We are also grateful for the helpful comments of the anonymous reviewers of several conference papers in which we presented our preliminary work.

This research has been supported by the Spanish Government, under projects TIC2000-0664-C02-02 and FIT-150500-2002-416.