A novel form of labelled transition system is proposed, wherethe labels are the arrows of a category, and adjacent labels in computations are required to be composable. Such transition systems provide thefoundations for modular SOS descriptions of programming languages.Three fundamental ways of transforming label categories, analogous tomonad transformers, are provided, and it is shown that their applicationspreserve computations in modular SOS. The approach is illustrated withfragments taken from a modular SOS for ML concurrency primitives
