Kreowski, Hans-Jörg, Fernando Orejas, and Hartmut Ehrig. 1997. “Correctness of Horizontal and Vertical Composition for Implementation Concepts Based on Constructors and Abstractor”. Revista Matemática Complutense 10 (2): 365. https://doi.org/10.5209/rev_REMA.1997.v10.n2.17446.