-------------------------------------------------------------------------------- IFIP Working Group 1.6 on Term Rewriting Report of the annual meeting: 18 July 2026 (Lisbon, Portugal, hybrid) Chair: Cynthia Kop Co-Chair/Secretary: Carsten Fuhs -------------------------------------------------------------------------------- AGENDA (1) Presentation by Clemens Grabmayer (2) Presentation by Misaki Kojima (3) Presentation by Vincent van Oostrom (4) Presentation by Uwe Waldmann (5) International School on Rewriting 2026 by Cynthia Kop (6) International School on Rewriting 2028 by Adrián Riesco Rodríguez (7) Rewriting Website by Luigi Liquori (8) Business Meeting (memberships, auxiliary topics) -------------------------------------------------------------------------------- (1) 9:00 Clemens Grabmayer Title: Finding small steps for approaching hard questions, in work on the process semantics of regular expressions Abstract: From my work on the process interpretation of regular expressions (some jointly with Wan Fokkink, and earlier with Jos Baeten and Flavio Corradini) I want to explain several examples for where my background in rewriting, proof theory, and Lambda Calculus was instrumental in finding a solution for an open question, or at least for better understanding a problem's difficulty. The hard questions hereby concern the recognition of graphs that are bisimilar to process interpretations of regular expressions, and Milner's axiomatisation question. The examples concern rewriting-inspired approaches rather than direct applications of rewriting theory. They will be of two sorts: (1): Formulating local simplifications of process graphs with structure constraints as small steps in order to reason about global properties (such as structure preservation/non-preservation under bisimulation collapse, decidability of expressibility by a regular expression). (2): Interpretation-readback correspondences, of varying tightness, between regular expressions (as terms) and graphs (denoting processes). (2) 10:30 Misaki Kojima Title: All-Path Reachability Analysis for Runtime-Error Verification Abstract: This work proposes a method for verifying runtime errors in concurrent programs using logically constrained term rewrite systems (LCTRSs). We present a method for transforming concurrent programs with semaphores into LCTRSs and reducing runtime-error verification to the all-path reachability (APR) problem. Furthermore, we propose proof methods for proving and disproving APR problems in the style of cyclic proofs. (3) 11:30 Vincent van Oostrom Title: Closures versus Freeness Abstract: In the literature one finds two views on rewriting: 1. as relations (Dershowitz & Jouannaud, Klop, Baader & Nipkow, Ohlebusch,...), and 2. as systems (Newman,Hindley, Melliès,Khasidashvili,...). We argue that it is interesting to systematically connect both views, supported by various simple examples. For instance, reducibility (the reflexive--transitive closure) in view 1 is connected to reduction (a certain free construction) in view 2. After discussing several such basic examples, we present the general pattern, with as basic intuition that view 1 corresponds to closure properties (to adjectives: reducibility, convertibility,...), and view 2 to free constructs (to nouns: reduction, conversion, ...), and conclude with discussing how this connexion is helpful, leads to new results and rewrite theory. (4) 13:45 Uwe Waldmann Title: Clause orderings: What's in between weight-based orderings and multiset extensions? Abstract: Theorem proving calculi like superposition are parametrized by term and clause orderings. These orderings are used to restrict the search space, to prove the calculi complete, and to show their compatibility with redundancy deletion and simplification techniques. Traditionally, clause orderings are defined as multiset extensions of multiset extensions of term orderings. These orderings justify almost all simplification techniques commonly found in today’s theorem provers, with one notable exception: They cannot be used to prove that superposition-like calculi are compatible with certain kinds of variable elimination. We discuss methods to partially overcome this problem and demonstrate the limits of these methods. (5) 14:45 Cynthia Kop Title: International School on Rewriting 2026 Cynthia Kop reported on the International School on Rewriting (ISR) 2026, which she organised from 12 to 16 July in Nijmegen, The Netherlands. The School had been scheduled for this period to make it possible for participants from other continents to participate in ISR and then to go to the Federated Logic Conferences (FLoC) in Lisbon from 18 July 2026 onwards without having to do several intercontinental flights. However, all attending participants were based in Europe. The overlap with the FLoC summer schools was considered less problematic based on the assumption that a student would attend only one summer school per year. The classes of ISR 2026 were taught in the Mercator building of RU Nijmegen. Accommodation was provided at a holiday park in Groesbeek, with about half an hour travel time by bus or bicycle between the two locations. As in many previous instances of ISR, two tracks were offered: a basic track providing an introduction into term rewriting and an advanced track on selected topics in rewriting taught by researchers active in these topics. Unfortunately, no external funding was available. Feedback by potential funders seemed to indicate that ISR was considered too small. As a consequence, several interested participants from Brazil were unable to attend ISR. There were 3 registrations (of which only 2 materialisaed) in the basic track and 6.5 registrations for the advnaced track (the half-registration was by a local volunteer who attended half the sessions). Participant split according to country of affiliation: 4.5 The Netherlands 2 Germany 1 France 1 Portugal The low level of available income was addressed partly by preparation of breakfast and dinner at the accommodation by the organisers and volunteers (rather than external catering) to keep expenses low. Special mention was given to volunteer student Mikhail and management assistant Hilde, who provided significant labour to ISR 2026. Both tracks showed very active participation by the students. The joint accommodation enabled great social cohesion, with board games, a swimming pool, a cinema for following the men's football World Cup, and joint cleaning activities, leading to new friendships. Specifically, the interest in rewriting actively increased. Certain aspects required a degree of improvisation: the Nijmegen marches scheduled to start after ISR leading to road closures, a noisy video projector needing to be dealt with, and the high temperatures and insolation causing a change of the excursion destination from the unshaded Stadswaard to the Museum De Bastei, a former fortress shielded from the sun by thick walls. (6) 15:00 Adrián Riesco Rodríguez (online) Title: International School on Rewriting 2028 Adrián Riesco Rodríguez presented a bid to host ISR 2028 at the Fukuoka conference facility affiliated with Kyushu University in Japan. The organising committee would include Daniel Gaina, Nao Hirokawa, Canh Minh Do, Kazuhiro Ogata, Adrián Riesgo, and Ionut Tutu. The facility is located 20 minutes from the airport in the Nishijin district, in a safe and welcoming area that is easy to navigate for international visitors. For the topics and lecturers, a mix of invited and open calls would be used. The School would be scheduled for late August / early September, when little overlap with related conference would be expected. The planned budget would involve costs per day of about EUR 75 - 105 including accommodation, leading to a reservation fee of about EUR 400. Cynthia Kop raised the point that FSCD was likely to run in late August/early September, indicating the need for coordination of the schedule with the organisers of FSCD 2028. Adrián Riesgo acknowledged this issue. René Thiemann, the Chair of the Steering Committee of ISR, thanked Adrián Riesgo for the bid and enquired whether the funding was bound for specific purposes, e.g., travel grants vs a general decrease of registration fees. Adrián Riesgo responded that the purpose would be to support speakers who would otherwise not be able to come, but other than that lower fees would be the goal. René Thiemann stated that the bylaws required a vote in the Business Meeting and that an answer would be provided in a few hours. (7) 15:45 Luigi Liquori Title: An update on the rewriting.inria.fr Website Project Luigi Liquori reminded the participants that the website https://rewriting.loria.fr was no longer maintained as Laurent Vigneron had moved to other topics and Nachum Dershowitz had given the floor to younger people. He mentioned the presentation he had given at the IFIP WG 1.6 meeting co-located with FSCD and CADE in Rome in 2023 on the effort to revitalise and modernise the https://rewriting.org website (which was an alias for the https://rewriting.loria.fr website). At the time, the modernised version was still a student project. As the goal was to deploy the website in the Inria cloud, professional help would be needed. He invested some of his funding and contracted the student company JMC Nice to bring the site to the required standard. Now the website at https://rewriting.inria.fr works well on phone, tablet, and PC browsers. The company provides three months of guarantee to fix bugs reported in this period. Special thanks go to Pascal Tempier, in change of administration on the Inria side. As a related topic, Luigi Liquori also discussed the rewriting mailing list rewriting@ens-lyon.fr with about 300 subscribers. The mailing list is currently hosted at ENS Lyon, yet the long-time maintainer Pierre Lescanne would like to hand over the mailing list due to his retirement. Luigi Liquori announced that a new mailing list for the rewriting community was set up at rewriting@inria.fr, with Frédéric Blanqui kindly donating the e-mail address. The planned migrating day to the new mailing list would be on 1 September 2026. Hélène Kirchner thanked Luigi Liquori for his efforts and pointed out that the old page was difficult to change. It would work well with community members' contributions. (8) 16:15 IFIP WG 1.6 Business meeting Attendees: Sandra Alves Takahito Aoto Mauricio Ayala-Rincón Maria Paola Bonacina Jörg Endrullis (online, could not join BM due to technical issues, but attempted to do so; present online during the presentations) Santiago Escobar Maribel Fernández (online) Carsten Fuhs Jürgen Giesl (online) Nao Hirokawa (online) Claude Kirchner (online) Hélène Kirchner (online) Cynthia Kop Temur Kutsia Luigi Liquori Christopher Lynch (online) Naoki Nishida Femke van Raamsdonk (online) René Thiemann The following points were discussed: 1. In 2026, Clemens Grabmayer and Uwe Waldmann had been invited to present their work for the second time to the IFIP WG 1.6 on Rewriting and accepted the invitation. According to the bylaws of the working group, this should trigger an invitation to both to join the working group. Many enthusiastic endorsements were provided for this proposal, and unanimous support was expressed. 2. Formally, the bid to host ISR 2028 at Fukuoka required approval by the business meeting. Sandra Alves pointed out that two proposals for FSCD 2028 in the suggested time period had been made and advised coordination. After short discussion, unanimous support for the bid was expressed by the working group. 3. The next topic was the new rewriting website that had earlier been presented by Luigi Liquori. Maria Paola Bonacina stated that the new structure and the improved infrastructure would make it much easier to populate the site and that the page would indeed be populated. Cynthia Kop suggested that members of the working group should sign up as admins so that they would be able to add their students, who would then also be able to contribute. Maria Paola Bonacina pointed out that the model for the site was crowd work and that everybody had a /little/ of time to put in there, leading to a good website. Santiago Escobar enquired whether it was possible to see who had made edits. Luigi Liquori confirmed that this feature was indeed present. 4. On other business, Hélène Kirchner thanked the organisers, in particular for providing the option to join the meeting online. No further business was discussed.