Zachar, Z. “The Solvability of the Equivalence Problem for Deterministic Frontier-to-Root Tree Transducers”. Acta Cybernetica 4, no. 2 (January 1, 1979): 167-177. Accessed May 3, 2024. https://cyber.bibl.u-szeged.hu/index.php/actcybern/article/view/3175.