1. Center for the Study of Language and Information, Stanford University, Stanford, CA, 94305-4101, U.S.A.
2. Instituto de Matematica e Estatística, Universidade do Estado do Rio de Janeiro, Rio de Janeiro, RJ, 20550-900, Brazil
3. Center for the Study of Language and Information, Stanford University, Stanford, CA 94305-4101, U.S.A.