Net Systems of Level 1

Condition/Event (C/E) Systems are Petri Net Systems of level 1 which require the net structure to be pure and simple. The semantics are defined by the full marking class (forward and backward reachability) and require 1-liveness.
Elementary Net (EN) Systems have been defined by G. Rozenberg and P. S. Thiagarajan as a more simpler model with only forward reachability.
1-safe systems belong to net systems of level 1, since their places are marked by at most one unstructured token. Nevertheless they are usually defined as a subclass of ordinary Petri Nets with unary arc weights, infinite place capacities and boolean markings.