Joachim Niehren Tim Priesnitz Zhendong Su Complexity of Subtype Satisfiability over Posets. 357-373 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=3444&spage=357 2005 conf/esop/2005 ESOP db/conf/esop/esop2005.html#NiehrenPS05