Stefano Bistarelli Philippe Codognet Yan Georget Francesca Rossi Abstracting Soft Constraints. 108-133 http://link.springer.de/link/service/series/0558/bibs/1865/18650108.htm 1999 conf/compulog/1999 New Trends in Constraints db/conf/compulog/compulog1999.html#BistarelliCGR99