Ehrenfeucht, Andrzej. “On Representing RE Languages by One-Sided Internal Contextual Languages”. Acta Cybernetica 12, no. 3 (January 1, 1996): 217-233. Accessed October 21, 2021. https://cyber.bibl.u-szeged.hu/index.php/actcybern/article/view/3458.