Phuapong, Sarawut, and Nuthawud Sungtong. “Some Algebraic Properties of Regular Tree Transformations”. Communications in Mathematics and Applications 10, no. 4 (December 31, 2019): 809–819. Accessed May 3, 2024. http://www.rgnpublications.com/journals/index.php/cma/article/view/1273.