Notes[ref]This paper is a summary of one of the sections in Alvarez and Peña (1995). The authors would like to acknowledge the financial support of the John D. and Catherine T. MacArthur Foundation. Thanks to Pedro Pablo Acosta, Armando Alvarez Dozáguez, Jorge F. Pérez-López, Ricardo A. Puerta, Alberto Ribalta and Federico Sulroca for the information […]