New email address robert.walters@uninsubria.it New mail address Universita' dell'Insubria Dipartimento di Scienze CC, FF e MM Via Valleggio 11 22100 COMO, Italy Web homepage http://www.unico.it/~walters/ My recent papers are available there: the latest being P. Katis, N. Sabadini, R.F.C. Walters Feedback, trace and fixed point semantics We introduce a notion of category with feedback-with-delay, closely related to the notion of traced monoidal category, and show that the Circ construction of [JPAA 115: 141--178, 1997] is the free category with feedback on a symmetric monoidal category. Combining with the Int construction of Joyal-Street-Verity [Math. Proc. Camb.Phil. Soc., 119, 447-468, 1996] we obtain a description of the free compact closed category on a symmetric monoidal category. We thus obtain a categorical analogue of the classical localization of a ring with respect to a multiplicative subset. In this context we define a notion of fixed-point semantics of a category with feedback which is seen to include a variety of classical semantics in computer science. Others: R. Rosebrugh, N. Sabadini, R. F. C. Walters, Minimization and Minimal Realization in Span(Graph), submitted P. Katis, N. Sabadini, RFC Walters, Classes of finite state automata for which compositional minimization is linear time Fabio Gadducci, Piergiulio Katis, Ugo Montanari, Nicoletta Sabadini, Robert F.C. Walters, Comparing cospan-spans and tiles via a Hoare-style process calculus