  A crucial question in analyzing a concurrent system is to determine its
long-run behaviour, and in particular, whether there are irreversible choices
in its evolution, leading into parts of the reachability space from which there
is no return to other parts. Casting this problem in the unifying framework of
safe Petri nets, our previous work has provided techniques for identifying
attractors, i.e. terminal strongly connected components of the reachability
space, whose attraction basins we wish to determine. Here, we provide a
solution for the case of safe Petri nets. Our algorithm uses net unfoldings and
provides a map of all of the system's configurations (concurrent executions)
that act as cliff-edges, i.e. any maximal extension for those configurations
lies in some basin that is considered fatal. The computation turns out to
require only a relatively small prefix of the unfolding, just twice the depth
of Esparza's complete prefix.
