Dolors Costal Cristina Gómez Anna Queralt Ruth Raventós Ernest Teniente Improving the definition of general constraints in UML. 469-486 2008 7 Software and System Modeling 4 http://dx.doi.org/10.1007/s10270-007-0078-4 db/journals/sosym/sosym7.html#CostalGQRT08