db/conf/krdb/Hustadt94.html Ullrich Hustadt Do we need the closed world assumption in knowledge representation? 1994 conf/krdb/94 KRDB db/conf/krdb/krdb94.html#Hustadt94 http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS/CEUR-WS-1/hustadt-long.ps KRDB1994/X08.pdf