Preface

This volume contains the proceedings of EXPRESS/SOS 2024, the Combined 31th International Workshop on Expressiveness in Concurrency (EXPRESS) and the 21th Workshop on Structural Operational Semantics (SOS).

The first edition of EXPRESS/SOS was held in 2012, when the EXPRESS and SOS communities decided to organise an annual combined workshop bringing together researchers interested in the formal semantics of systems and programming concepts, and in the expressiveness of computational models. Since then, EXPRESS/SOS was held as one of the affiliated workshops of the International Conference on Concurrency Theory (CONCUR). Following this tradition, EXPRESS/SOS 2024 was held affiliated to CONCUR 2024, as part of CONFEST 2024, in Calgary, Canada.

The topics of interest for the EXPRESS/SOS workshop include:

• expressiveness and rigorous comparisons between models of computation;
• expressiveness and rigorous comparisons between programming languages and models;
• logics for concurrency; analysis techniques for concurrent systems;
• comparisons between structural operational semantics and other formal semantic approaches;
• applications and case studies of structural operational semantics;
• software tools that automate, or are based on, structural operational semantics.

This volume contains revised versions of the 5 full papers, selected by the Program Committee, as well as the abstract of the invited presentation by Clément Aubert.

We would like to thank the authors of the submitted papers, the invited speaker, the members of the program committee, and their subreviewers for their contribution to both the meeting and this volume. We also thank the CONFEST 2024 organising committees for hosting the workshop. Finally, we would like to thank our EPTCS editor Rob van Glabbeek for publishing these proceedings and his help during the preparation.

Georgiana Caltais and Cinzia Di Giusto, October 2024

Program Committee

• Elli Anastasiadi, Uppsala University, Sweden
• Matteo Cimini, University of Massachusetts Lowell, USA
• Wan Fokkink, Vrije Universiteit Amsterdam, The Netherlands
• Adrian Francalanza, University of Malta
• Fatemeh Ghassemi, University of Tehran, Iran
• Lorenzo Gheri, University of Liverpool
• Eva Graversen, University of Southern Denmark, Denmark
• Jean Krivine, CNRS, Paris, France
• Sergueï Lenglet, Université de Lorraine
• Doriana Medic, University of Turin, Italy
• Maurizio Murgia, Gran Sasso Science Institute
• António Ravara, Universidade NOVA de Lisboa, Portugal
• Marjan Sirjani, Malardalen University, Sweden
• Felix Stutz, University of Luxembourg, Luxembourg
• Emilio Tuosto, Gran Sasso Science Institute, Italy
• Frank Valencia, LIX, Ecole Polytechnique, France
• Daniele Varacca, LACL - Université Paris Est Créteil, France
• Gianluigi Zavattaro, Department of Computer Science and Engineering - University of Bologna, Italy

Reverse My Computation? But Why?

A typical computer user knows the difference between what can be undone on a computer, and what cannot. They may be familiar with the "undo" feature of text editors but understand the impossibility of recovering an unsaved document after an emergency shutdown. Creating programs guaranteeing that any action can be undone requires to design and implement reversible programming languages. While such languages come with interesting built-in security features (because any past action can be investigated), they also raise challenges when it comes to concurrency. Indeed, undoing an action that involved synchronization between multiple actors requires all actors to agree to undo said action. Process algebras offer an interesting frame to study reversible computation, and reciprocally. Enriching process algebras such as CCS with memory, identifiers or keys, allowed to represent reversible computation, and in turn helped gained a finer understanding of causality, bisimulations, and other "true concurrency" notions. This talk offers to briefly motivate the interests of reversible computation, and to discuss the new lights it shed on process algebras.