Our interest in the questions addressed in this paper was raised by past discussions with Félix Costa and Cris Moore. This work was partially supported by Fundaçã o para a Ciência e a Tecnologia (FCT) and FEDER via the Center for Logic and Computation – CLC, the project ConTComp POCTI/MAT/45978/2002 and grant SFRH/BD/17436/2004. Additional support was also provided by the Fundaçã o Calouste Gulbenkian through the Programa Gulbenkian de Estímulo à Investigaçã o.