Jianfeng Lu Ruixuan Li Zhengding Lu Jinwei Hu Xiaopu Ma Specification and Enforcement of Static Separation-of-Duty Policies in Usage Control. 403-410 2009 ISC http://dx.doi.org/10.1007/978-3-642-04474-8_32 conf/isw/2009 db/conf/isw/isc2009.html#LuLLHM09