Fülöp, Zoltán, and Sándor Vágvölgyi. “Results on Compositions of Deterministic Root-to-Frontier Tree Transformations”. Acta Cybernetica 8, no. 1 (January 1, 1987): 49-61. Accessed April 16, 2024. https://cyber.bibl.u-szeged.hu/index.php/actcybern/article/view/3320.