Chaoyi Pang Xiuzhen Zhang Yanchun Zhang Kotagiri Ramamohanarao The Efficient Maintenance of Access Roles with Role Hiding. 139-149 2008 COMAD http://www.cse.iitb.ac.in/~comad/2008/PDFs/13.pdf conf/comad/2008 db/conf/comad/comad2008.html#PangZZR08