Dassow, Jürgen, Alexandru Mateescu, G. Paun, and A. Salomaa. 1992. “Regularizing Context-Free Languages by AFL Operations : Concatenation and Kleene Closure”. Acta Cybernetica 10 (4):243-53. https://cyber.bibl.u-szeged.hu/index.php/actcybern/article/view/3410.