In: Rozenberg, G.: Lecture Notes in Computer Science, Vol. 340: Advances in Petri Nets 1988, pages 44-61. Springer-Verlag, 1988.
Abstract: Subnets whose relative boundary consists only of transitions, termed building blocks, are used to structure specifications of distributed systems. In this paper the problem is considered to determine the effects of a building block on its possible environments, considering only its internal structure. External equivalence of building blocks allows one to change these components without affecting the behaviour of the environment.