Michele Boreale Davide Sangiorgi Some Congruence Properties for Pi-Calculus Bisimilarities. 159-176 1998 198 Theor. Comput. Sci. 1-2 db/journals/tcs/tcs198.html#BorealeS98 http://dx.doi.org/10.1016/S0304-3975(97)00125-4