Stefano Bistarelli Ugo Montanari Francesca Rossi Thomas Schiex Gérard Verfaillie Hélène Fargier Semiring-Based CSPs and Valued CSPs: Frameworks, Properties, and Comparison. 199-240 1999 4 Constraints 3 db/journals/constraints/constraints4.html#BistarelliMRSVF99