On Term Graphs as an Adhesive Category

Abstract

The recent interest in bisimulation congruences for reduction systems, stimulated by the research on general (often graphical) frameworks for nominal calculi, has brought forward many proposals for categorical formalisms where relevant properties of observational equivalences could be auto- matically verified.Interestingly, some of these formalisms also identified suitable categories where the standard tools and techniques developed for the double-pushout approach to graph transformation [A. Corradini, U. Montanari, F. Rossi, H. Ehrig, R. Heckel and M. Löwe, Algebraic approaches to graph transformation I: Basic concepts and double pushout approach, in: G. Rozenberg, editor, Handbook of Graph Grammars and Computing by Graph Transformation, I: Foundations, World Scientific, 1997, pp. 163–246] could be recast, thus providing a valid alternative to the High-Level Replacement Systems paradigm [H. Ehrig, A. Habel, H.-J. Kreowski and F. Parisi-Presicce, Parallelism and concurrency in highlevel replacement systems, Mathematical Structures in Computer Science 1 (1991) 361–404].In this paper we consider the category of term graphs, and we prove that it (partly) fits in the general framework for adhesive categories, developed in [S. Lack, and P. Sobociński, Adhesive categories, in: I. Walukiewicz, editor, Foundations of Software Science and Computation Structures, Lect. Notes in Comp. Sci. 2987 (2004), pp. 273–288, P. Sobociński, “Deriving bisimulation congruences from reduction systems”, Ph.D. thesis, BRICS, Department of Computer Science, University of Aaurhus (2004)], extended in [H. Ehrig, A. Habel, J. Padberg and U. Prange, Adhesive high-level replacement categories and systems, in: G. Engels and F. Parisi-Presicce, editors, Graph Transformation, Lect. Notes in Comp. Sci. (2004)] and applied to reduction systems in [V. Sassone, and P. Sobociński, Congruences for contextual graph-rewriting, Technical Report RS-04-11, BRICS, Department of Computer Science, University of Aarhus (2004)]. The main technical achievement concerns the proof that the category of term graphs is actually quasi-adhesive, obtained by proving the existence of suitable Van Kampen squares.