Mahesh V. Tripunitara Ninghui Li A theory for comparing the expressive power of access control models. 231-272 2007 15 Journal of Computer Security 2 http://iospress.metapress.com/openurl.asp?genre=article&issn=0926-227X&volume=15&issue=2&spage=231 db/journals/jcs/jcs15.html#TripunitaraL07