Arbib, Michael Anthony, and Ernest Gene Manes. “Tree Transformations and the Semantics of Loop-Free Programs”. Acta Cybernetica 4, no. 1 (January 1, 1978): 11-17. Accessed April 17, 2024. https://cyber.bibl.u-szeged.hu/index.php/actcybern/article/view/3161.