Validating QBF Validity in HOL4

The Quantified Boolean Formulae (QBF) solver Squolem can generate certificates of validity, based on Skolem functions. We present independent checking of these certificates in the HOL4 theorem prover. This enables HOL4 users to benefit from Squolem’s automation for valid QBF problems. Detailed performance data shows that LCF-style checking of validity certificates is often (but not always) feasible even for large QBF instances. Additionally, our work provides high correctness assurances for Squolem’s claims of validity and uncovered a soundness bug in a previous version of its certificate validator QBV.

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