Yan Zhang Yi Liu Le Zhang Zhiyi Ma Hong Mei Modeling and Checking for Non-functional Attributes in Extended UML Class Diagram. 100-107 2008 COMPSAC http://doi.ieeecomputersociety.org/10.1109/COMPSAC.2008.72 conf/compsac/2008 db/conf/compsac/compsac2008.html#ZhangLZMM08