Steinby, Magnus. “On DR Tree Automata, Unary Algebras and Syntactic Path Monoids”. Acta Cybernetica 23, no. 1 (January 1, 2017): 159-174. Accessed August 15, 2022. https://cyber.bibl.u-szeged.hu/index.php/actcybern/article/view/3920.