Kreowski, Hans-Jörg, Fernando Orejas, and Hartmut Ehrig. “Correctness of Horizontal and Vertical Composition for Implementation Concepts Based on Constructors and Abstractor”. Revista Matemática Complutense 10, no. 2 (January 1, 1997): 365. Accessed July 29, 2026. https://revistas.ucm.es/index.php/REMA/article/view/REMA9797230365A.