Sergio Guadarrama Susana Muñoz-Hernández Claudio Vaucheret Fuzzy Prolog: a new approach using soft constraints propagation. 127-150 2004 144 Fuzzy Sets and Systems 1 http://dx.doi.org/10.1016/j.fss.2003.10.017 db/journals/fss/fss144.html#GuadarramaMV04