Fabio Alessi Franco Barbanera Mariangiola Dezani-Ciancaglini 17-33 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=3085&spage=17 2003 conf/types/2003 TYPES db/conf/types/types2003.html#AlessiBD03