A Generic Tableau Prover and Its Integration with Isabelle
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This document describes a generic tableau prover that has been integrated with the Isabelle proof assistant. The prover uses free-variable tableau with iterative deepening and can handle higher-order logic. It translates Isabelle goals and proofs into a format the prover can handle, such as using standard Skolemization instead of dual Skolemization. Evaluation shows it outperforms an existing first-order prover in Isabelle on many benchmarks, taking 10 times less time for some proofs, while remaining compatible with Isabelle's tools. Its limitations include difficulty proving some set theory problems.
A Generic Tableau Prover and Its Integration with Isabelle
- 1. A Generic Tableau Prover and Its Integration with Isabelle Lawrence C. Paulson Computer Laboratory University of Cambridge 1
- 2. Overview of Isabelle • a generic interactive prover for FOL, set theory, HOL, . . . • Prolog influence: resolution of generalized Horn clauses Existing classical reasoner (Fast tac) • tableau methods • generic: accepts supplied rules • runs on Isabelle’s Prolog engine (trivial integration) 2
- 3. Objectives for the New Tactic • Genericity: no restriction to predicate logic • Power: quantifier duplication, transitivity reasoning . . . • Speed: perhaps 10–20 seconds for interactive use • Compatibility with Isabelle’s existing tools (Fast tac) 3
- 4. Why Write a New Tableau Prover? Q. Why not rewrite with A ⊆ B ⇐⇒ ∀x (x ∈ A → x ∈ B)? A. Destroys legibility A. Not always possible: inductive definitions Q. Why not just call Otter, SETHEO or LeanTaP? A. We need higher-order syntax 4
- 5. Typical Generic Tableau Rules type α type γ/β type δ/α t ∈ A ∩ B t ∈ A t ∈ B A ⊆ B ¬(?x ∈ A) | ?x ∈ B ¬(A ⊆ B) s ∈ A ¬(s ∈ B) Complications from genericity: • overloading store some type info • variable instantiation heuristic limits • recursive rules ad-hoc checks 5
- 6. Prover Architecture Free-variable tableau with iterative deepening (leanTaP) Term data structure: no types; variables as pointers Basic heuristics • discrimination nets • search-space pruning • delayed use of unsafe rules (γ-rules) • suppressing needless duplication 6
- 7. Integration I: Translating Isabelle Rules • multiple goal formulas via negation • dual Skolemization ⇒ standard Skolemization • simplification of higher-order conclusions (η-contraction) • limitations on function variables • type translation for overloading 7
- 8. Integration II: Translating Tableau Proofs Isabelle checks the proof—often the slowest phase • direct correspondence from proof steps to Isabelle tactics • failure might be caused by – breakdown of the correspondence – type complications • recomputation of unifiers • fancy tricks not possible (e.g. liberalized δ-rule) 8
- 9. Results & Limitations Good performance on first-order benchmarks e.g. Pelletier’s Mostly compatible with fast_tac; can be 10 times faster • and proves more theorems • but slower for some ‘obvious’ problems Set theory challenge: (∀x, y ∈ S x ⊆ y) → ∃z S ⊆ {z} 9
- 10. Conclusions • the first tableau prover with higher-order syntax? • the first tableau prover for ZF, HOL, inductive definitions, . . . ? • has almost replaced fast_tac • a good example of integration in daily use 10