Ian P. Gent Enrico Giunchiglia Massimo Narizzano Andrew G. D. Rowley Armando Tacchella Watched Data Structures for QBF Solvers. 25-36 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=2919&spage=25 2003 conf/sat/2003 SAT db/conf/sat/sat2003.html#GentGNRT03