@article{TE97,
title = {Hierarchies of Petri Net Languages and a Super-Normal Form },
author = {Tiplea, Ferucio Laurentiu and Ene, Cristian},
year = {1997},
journal = {Journal of Automata, Languages and Combinatorics},
number = {3},
pages = {187-208},
volume = {2},
team = {DCS,PACSS},
}