On the Concatenation of Infinite Traces

There is a straightforward generalization of traces to infinite traces as dependence graphs where every vertex has finitely many predecessors, or what is the same, as a backward closed and directed set of traces with respect to prefix ordering. However, this direct approach has a drawback since it allows no good notion of a concatenation. We solve this problem by adding to an infinite trace a second component. This second component is a finite alphabetic information which is called the alphabet at infinity. We obtain a compact and complete ultra-metric space where the concatenation is uniformly continuous and where the set of finite traces is an open, discrete, and dense subset. Our objects arise in a natural way from the consideration of dependence graphs where the induced partial order is well-founded. Such a graph splits into a so-called real part and a transfinite (or imaginary) part. From the transfinite part only the alphabet is of importance. Our approach is a non-trivial generalization of the well-known construction for direct products of free monoids and yields a convenient semantics for infinite concurrent processes.