QuBE++: An Efficient QBF Solver

In this paper we describe QUBE++, an efficient solver for Quantified Boolean Formulas (QBFs). To the extent of our knowledge, QUBE++ is the first QBF reasoning engine that uses lazy data structures both for unit clauses propagation and for pure literals detection. QUBE++ also features non-chronological backtracking and a branching heuristic that leverages the information gathered during the backtracking phase. Owing to such techniques and to a careful implementation, QUBE++ turns out to be an efficient and robust solver, whose performances exceed those of other state-of-the-art QBF engines, and are comparable with the best engines currently available on SAT instances.