Free Choice Petri Nets without frozen tokens and Bipolar Synchronization   Systems
  Bipolar synchronization systems (BP-systems) constitute a class of coloured Petri nets, well suited for modeling the control flow of discrete, dynamical systems. Every BP-system has an underlying ordinary Petri net, which is a T-system. Moreover, it has a second ordinary net attached, which is a free-choice system. We prove that a BP-system is live and safe if the T-system and the free-choice system are live and safe and if the free-choice system has no frozen tokens. This result is the converse of a theorem of Genrich and Thiagarajan and proves an elder conjecture. The proof compares the different Petri nets by Petri net morphisms and makes use of the classical theory of free-choice systems 
Joachim Wehler