Alimonti, P., E. Feuerstein, L. Laura, and U. Nanni. “Linear Time Analysis of Properties of Conflict-Free and General Petri Nets”. Department of Computer and System Sciences Antonio Ruberti Technical Reports, vol. 2, no. 9, July 2010, p. 24, https://rosa.uniroma1.it/rosa00/index.php/dis_technical_reports/article/view/8890.