M. Schiffers, and H. Wedde. Mathematical Foundations of Computer Science 1978, volume 64 of Lecture Notes in Computer Science, Springer Berlin Heidelberg, (1978)
C. Petri. Net theory and applications : Proceedings of the advanced course on general net theory,processes and systems (Hamburg, 1979), volume 84 of Lecture Notes in Computer Science, page 1--20. Springer-Verlag, Berlin, (1980)