Ulrich Schiel. 1990.
 Redes de Petri Hierárquicas. In <em>Proceedings of the 4th Brazilian Symposium on Software Engineering</em>, October 24, 1990, Águas de São Pedro/São Paulo, Brasil. SBC, Porto Alegre, Brasil, 124-133. DOI: <a href="https://doi.org/10.5753/sbes.1990.24171">https://doi.org/10.5753/sbes.1990.24171</a>.