Hans Kleine Büning K. Subramani Xishun Zhao On Boolean Models for Quantified Boolean Horn Formulas. 93-104 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=2919&spage=93 2003 conf/sat/2003 SAT db/conf/sat/sat2003.html#BuningSZ03