Ninghui Li Qihua Wang Wahbeh H. Qardaji Elisa Bertino Prathima Rao Jorge Lobo Dan Lin Access control policy combining: theory meets practice. 135-144 2009 SACMAT http://doi.acm.org/10.1145/1542207.1542229 conf/sacmat/2009 db/conf/sacmat/sacmat2009.html#LiWQBRLL09