Manuel Hilty Alexander Pretschner David A. Basin Christian Schaefer Thomas Walter A Policy Language for Distributed Usage Control. 531-546 2007 conf/esorics/2007 ESORICS http://dx.doi.org/10.1007/978-3-540-74835-9_35 db/conf/esorics/esorics2007.html#HiltyPBSW07