Moving Definition Variables in Quantified Boolean Formulas

Augmenting problem variables in a quantified Boolean formula with definition variables enables a compact representation in clausal form. Generally these definition variables are placed in the innermost quantifier level. To restore some structural information, we introduce a preprocessing technique that moves definition variables to the quantifier level closest to the variables that define them. We express the movement in the QRAT proof system to allow verification by independent proof checkers. We evaluated definition variable movement on the QBFEVAL' 20 competition benchmarks. Movement significantly improved performance for the competition's top solvers. Combining variable movement with the preprocessor BLOQQER improves solver performance compared to using BLOQQER alone.