Angelo, Monti, and Adriano Peron. “Logical Definability of Y-Tree and Trellis Systolic ω-Languages”. Acta Cybernetica 15, no. 1 (January 1, 2001): 75-100. Accessed October 21, 2021. https://cyber.bibl.u-szeged.hu/index.php/actcybern/article/view/3564.