All arcs have multiplicity one (i.e., are unity weighted).
All transitions sharing a common input place have no other input place.
All transitions sharing a common input place have the same input places.
Every transition has exactly one input place and exactly one output place.
Every place has exactly one input transition and exactly one output.
There is an undirected path between every two nodes (places or transitions).
There is a directed path between every two nodes (places or transitions).
One or more places have no input transitions.
One or more places have no output transitions.
One or more transitions have no input places.
One or more transitions have no output places.
No transition has an input place that is also an output place.
For each transition, the sum of valuations (i.e., "inscriptions" in PNML terminology)
for all input arcs equals the sum of valuations for all output arcs.
Note : if there are no "multiple" arcs, the valuation of each arc is equal to one.
For each transition, the sum of valuations for all input arcs equals or exceeds the sum of valuations for all output arcs.
Places are structured into hierarchically nested sequential units.
The definition of Nested-Unit Petri Nets (NUPN) is available from http://mcc.lip6.fr/nupn.php
In every reachable marking, there is no more than one token on each place.
There exists a reachable marking from which no transition can be fired.
From every reachable marking, there is a transition path going back to the initial marking.
For every transition, there exists a reachable marking in which the transition can fire.
For every transition, from every reachable marking, one can reach a marking in which the transition can fire.