Toni Jussila Armin Biere Carsten Sinz Daniel Kröning Christoph M. Wintersteiger A First Step Towards a Unified Proof Checker for QBF. 201-214 2007 conf/sat/2007 SAT http://dx.doi.org/10.1007/978-3-540-72788-0_21 db/conf/sat/sat2007.html#JussilaBSKW07