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arxiv: 2605.15390 · v1 · pith:3NGDADBHnew · submitted 2026-05-14 · 💻 cs.LO · cs.FL

Kofola 1.0: A Modular Approach to {ω}-Regular Complementation and Inclusion Checking (Technical Report)

Pith reviewed 2026-05-19 15:14 UTC · model grok-4.3

classification 💻 cs.LO cs.FL
keywords Büchi automatacomplementationinclusion checkingstrongly connected componentsmodular frameworkemptiness checkingomega-regular languagesmodel checking
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The pith

Kofola decomposes Büchi automata into strongly connected components and applies tailored complementation to each.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper presents Kofola as a tool for complementing Büchi automata and checking inclusion between them, operations central to verifying systems that run indefinitely. It breaks the input automaton into its strongly connected components and selects a complementation procedure matched to the structure of each component rather than applying one uniform method. A new on-the-fly emptiness checker for the generalized Rabin pair condition lets the process stop once enough states have been explored. If the approach holds, verification tasks in model checking and monitoring can scale to larger automata without exhausting time or memory.

Core claim

Kofola implements a modular complementation framework that decomposes the input automaton into strongly connected components and applies to each component a complementation algorithm tailored to its structural properties. Building on this modular construction, Kofola also provides modular inclusion checking with new heuristics. A key ingredient is a new on-the-fly emptiness-checking algorithm for the simple generalized Rabin pair condition produced by our complementation, allowing the search to terminate as soon as the explored state space suffices.

What carries the argument

The modular complementation framework that decomposes the automaton into strongly connected components and applies structure-specific algorithms, supported by an on-the-fly emptiness checker for the resulting generalized Rabin pair condition.

If this is right

  • Complementation of automata from real verification problems becomes feasible at larger sizes.
  • Inclusion checks between automata run faster through reuse of the same modular structure and added heuristics.
  • Verification tools can explore more complex omega-regular properties without early termination.
  • The produced generalized Rabin conditions can be handled directly in downstream emptiness checks without full expansion.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The same SCC decomposition could be reused to speed up other automata operations such as equivalence checking or minimization.
  • Adding new component-specific algorithms would require only local changes to the framework rather than a full redesign.
  • The observed robustness suggests that structure-aware methods may generalize to nondeterministic automata beyond the Büchi case.

Load-bearing premise

The decomposition into strongly connected components combined with per-component tailored algorithms and the on-the-fly emptiness checker preserves correctness while delivering efficiency gains.

What would settle it

A Büchi automaton for which Kofola produces a complement that accepts a word the original rejects, or a benchmark suite from practical applications where Kofola times out or uses more memory than non-modular tools.

Figures

Figures reproduced from arXiv: 2605.15390 by Luk\'a\v{s} Hol\'ik, Nicolas Mazzocchi, Ondrej Alexaj, Ond\v{r}ej Leng\'al, Vojt\v{e}ch Havlena, Yong Li.

Figure 1
Figure 1. Figure 1: Architecture of Kofola Kofola is implemented in C++ and avail￾able at [2] under the GNU GPL 3.0 license. Its architecture is shown in Section 2. The Frontend uses Spot [21] to read input au￾tomata A1.hoa and, for inclusion checking only, A2.hoa, both in the HOA [7] format. Next, the automata enter Preprocessing and setup, where (i) Spot reduces them using ei￾ther Low or High reduction level, depending on t… view at source ↗
Figure 2
Figure 2. Figure 2: Visualization of macrostate’s succes￾sor computation over symbol a. The input of the top-level algorithm are, except the automaton itself, also partitions P1, . . . , Pk of the input au￾tomaton and the partial algorithms AlgP1 , . . . , AlgPk . The macrostates of the top-level algoritm are then of the form (S, M1, . . . , Mk), where S is the set of reachable states and M1, . . . , Mk are macrostates of the… view at source ↗
Figure 3
Figure 3. Figure 3: Comparison of state counts of complement automata generated by [PITH_FULL_IMAGE:figures/full_fig_p011_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Comparison of runtime of inclusion checking for [PITH_FULL_IMAGE:figures/full_fig_p012_4.png] view at source ↗
read the original abstract

We present Kofola, an efficient tool for complementation and inclusion checking of B\"uchi automata, two central tasks in automata-theoretic verification with applications in model checking, monitoring, and theorem proving. Kofola implements a state-of-the-art modular complementation framework that decomposes the input automaton into strongly connected components and applies to each component a complementation algorithm tailored to its structural properties. Building on this modular construction, Kofola also provides modular inclusion checking with new heuristics. A key ingredient is a new on-the-fly emptiness-checking algorithm for the simple generalized Rabin pair condition produced by our complementation, allowing the search to terminate as soon as the explored state space suffices. Empirical evaluation shows that Kofola is highly competitive with state-of-the-art complementation and inclusion-checking tools: it is the most robust tool in our evaluation and often outperforms competitors by several orders of magnitude on benchmarks from practical applications.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

1 major / 2 minor

Summary. The paper presents Kofola 1.0, a tool for complementation and inclusion checking of Büchi automata. It implements a modular framework that decomposes the input automaton into strongly connected components (SCCs) and applies a complementation algorithm tailored to the structural properties of each component. The approach is extended to modular inclusion checking with new heuristics. A central technical contribution is a new on-the-fly emptiness-checking algorithm for the simple generalized Rabin pair condition generated by the per-SCC complementation, which permits early termination once the explored state space is sufficient. Empirical evaluation on benchmarks from practical applications indicates that Kofola is the most robust tool in the comparison and frequently outperforms existing state-of-the-art tools by several orders of magnitude.

Significance. If the modular decomposition preserves correctness and the on-the-fly emptiness checker is shown to be sound for the Rabin-pair conditions arising from SCC-specific complementation, the work would provide a practically significant advance in automata-theoretic verification. The combination of structural decomposition with a specialized emptiness procedure directly targets the performance bottlenecks in complementation and inclusion, which are core operations in model checking, monitoring, and theorem proving. Reproducible tool implementation and evaluation on application-derived benchmarks would further strengthen the contribution.

major comments (1)
  1. [emptiness-checking algorithm section] The section describing the on-the-fly emptiness checker for generalized Rabin pairs must supply a complete argument that the early-termination criterion remains sound when the acceptance condition is produced by per-SCC complementation. In particular, it is necessary to show that no accepting run can exist outside the current exploration frontier even if the Rabin pairs introduced by individual SCC complements depend on paths that cross original component boundaries. Without this argument, both the claimed correctness and the reported speed-ups rest on an unverified assumption.
minor comments (2)
  1. [abstract and introduction] The abstract and introduction should include a brief pointer to the specific theorem or lemma that establishes correctness of the modular construction.
  2. [empirical evaluation] Benchmark tables would benefit from explicit reporting of the number of timeouts and memory-outs for each tool to substantiate the robustness claim.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for the detailed and constructive review. The positive assessment of the practical significance of Kofola 1.0 is appreciated. We address the single major comment below and will revise the manuscript to strengthen the presentation.

read point-by-point responses
  1. Referee: [emptiness-checking algorithm section] The section describing the on-the-fly emptiness checker for generalized Rabin pairs must supply a complete argument that the early-termination criterion remains sound when the acceptance condition is produced by per-SCC complementation. In particular, it is necessary to show that no accepting run can exist outside the current exploration frontier even if the Rabin pairs introduced by individual SCC complements depend on paths that cross original component boundaries. Without this argument, both the claimed correctness and the reported speed-ups rest on an unverified assumption.

    Authors: We agree that an explicit, self-contained soundness argument for the early-termination criterion is required when the generalized Rabin pairs originate from the modular per-SCC complementation. Although the overall construction composes the per-SCC complements in a manner that keeps the acceptance condition local to the explored states (because each SCC complement is self-contained and the on-the-fly search respects the original transition structure), the manuscript currently presents the emptiness procedure at a high level without spelling out the cross-boundary case. In the revised version we will add a dedicated lemma (or subsection) that proves: (i) any accepting run of the complemented automaton must eventually enter and remain in one of the complemented SCCs, and (ii) the Rabin-pair condition for that SCC can be satisfied only by states reachable within the current exploration frontier, independent of unvisited paths that cross original component boundaries. This addition will also clarify why the reported speed-ups are sound. We therefore mark this revision as necessary. revision: yes

Circularity Check

0 steps flagged

No circularity: algorithmic framework and empirical results are self-contained

full rationale

The paper describes a modular complementation construction for Büchi automata that decomposes into SCCs and applies per-component algorithms, plus an on-the-fly emptiness checker for the resulting generalized Rabin conditions. These steps are standard automata-theoretic constructions and implementation choices whose correctness rests on established theory rather than any equation that reduces by definition to a fitted parameter, self-citation chain, or renamed input. Performance claims are supported by direct benchmarking against external tools on practical instances, with no load-bearing derivation that collapses to the paper's own outputs.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The contribution rests on standard automata-theoretic background rather than new free parameters, ad-hoc axioms, or invented entities.

axioms (1)
  • standard math Standard definitions and closure properties of Büchi automata and ω-regular languages hold.
    The modular complementation framework is built directly on these established properties.

pith-pipeline@v0.9.0 · 5725 in / 1312 out tokens · 45460 ms · 2026-05-19T15:14:45.565041+00:00 · methodology

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