Fabio Alessi Mariangiola Dezani-Ciancaglini Furio Honsell Filter Models and Easy Terms. 17-37 2001 conf/ictcs/2001 ICTCS http://link.springer.de/link/service/series/0558/bibs/2202/22020017.htm db/conf/ictcs/ictcs2001.html#AlessiDH01