1. E. Best: Structure Theory of Petri Nets: the Free Choice Hiatus. Advanced Course on Petri Nets, Bad Honnef (September 1986), to appear in Springer Lecture Notes in Computer Science.
2. E. Best and C. Fernández: Notations and Terminology on Petri Net Theory. Arbeitspapiere der GMD No.195 (1986).
3. F. Commoner: Deadlocks in Petri Nets. Report, Applied Data Inc., CA-7206-2311 (1972).
4. F. Commoner, A.W Holt, S. Even and A. Pnueli: Marked Directed Graphs. JCSS Vol.5, 511–523 (1971).
5. J. Desel: A Structural Property of Free Choice Systems. Petri Net Newsletters No.25, 16–20 (December 1986).