  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
