Efficient extraction of Skolem functions from QRAT proofs

Many synthesis problems can be solved by formulating them as a quantified Boolean formula (QBF). For such problems, a mere true/false answer is often not enough. Instead, expressing the answer in terms of Skolem functions reflecting the quantifier dependencies of the variables is required. Several approaches have been presented to extract such functions from term-resolution proofs. However, not all solvers and preprocessors are able to produce term-resolution proofs, especially when universal expansion is involved. In previous work, we developed the QRAT proof system consisting of three simple rules which allowed us to overcome this issue and to equip modern expansion-based tools like the preprocessor bloqqer with proof tracing. In this paper, we show how to extract Skolem functions from QRAT proofs. We present a general extraction tool and compare its performance to similar resolution-based tools. We show that the Skolem functions extracted from QRAT proofs are smaller than those produced by alternative approaches making our method in particular useful for synthesis applications.