Cheun Ngen Chong Sandro Etalle Pieter H. Hartel Comparing Logic-Based and XML-Based Rights Expression Languages. 779-792 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=2889&spage=779 2003 conf/otm/2003 OTM Workshops db/conf/otm/otm2003.html#ChongEH03