An Isabelle/HOL Framework for Synthetic Completeness Proofs

Proof assistants like Isabelle/HOL provide the perfect opportunity to develop more than just one-off formalizations, but frameworks for developing new results within a given area. Completeness results for logical calculi often include bespoke versions of Lindenbaum's lemma. In this paper, I mechanize an abstract, transfinite version of the lemma and use it to build witnessed, maximal consistent sets (MCSs) for any notion of consistency that satisfies a few requirements. I prove abstract results about when MCSs reflect the proof rules of the underlying calculi. Finally, I formalize a process for mechanically calculating saturated set conditions from a given logic's semantics. This separates the truth lemma that connects MCS-membership and satisfiability into semantic and syntactic components, giving concrete proof obligations for each operator of the logic. To illustrate the framework's applicability, I instantiate it with propositional, first-order, modal and hybrid logic examples. I mechanize strong completeness for each logic, even for uncountably large languages, proving that, if a formula is valid under a set of assumptions, then we can derive it from a finite subset.