Imreh, Balázs, and Magnus Steinby. “Directable Nondeterministic Automata”. Acta Cybernetica 14, no. 1 (January 1, 1999): 105-115. Accessed October 7, 2022. https://cyber.bibl.u-szeged.hu/index.php/actcybern/article/view/3514.