Department of Informatics, Faculty of Sciences, University of Lisbon, 1749-016 Lisbon, Portugal. dfaria@xldb.di.fc.ul.pt|Department of Informatics, Faculty of Sciences, University of Lisbon, 1749-016 Lisbon, Portugal. dfaria@xldb.di.fc.ul.pt|Department of Informatics, Faculty of Sciences, University of Lisbon, 1749-016 Lisbon, Portugal. dfaria@xldb.di.fc.ul.pt