Pöschel, Reinhard, Aleksander Semigrodskij, and Heiko Vogler. “Relationally Defined Clones of Tree Functions Closed under Selection or Primitive Recursion”. Acta Cybernetica 16, no. 3 (January 1, 2004): 411-425. Accessed December 22, 2024. https://cyber.bibl.u-szeged.hu/index.php/actcybern/article/view/3632.