The promised companion to the paper I announced last week is now available. The first version, which I gave to some people, actually CONTAINED AN ERROR --- so please download this version if you have an old copy. The file is CCPS.ps, and can be downloaded either from triples.math.mcgill.ca, or from theory.doc.ic.ac.uk. Regards, -- Dusko CONVENIENT CATEGORIES OF PROCESSES AND SIMULATIONS by Dusko Pavlovic Abstract. We show that irredundant trees, used by Dana Scott in the early sixties, can be used as canonical representants of the bisimilarity classes of automata (or of transition systems). Simulations then boil down to tree morphisms. Along the same lines, the categories of processes modulo the observational and the branching congruences, with the suitable simulations as morphisms again, are shown to be isomorphic with certain subcategories of the category of irredundant trees.