1. Andréka, H., Burmeister, P., Németi, I.: Quasivarieties of Partial Algebras — A Unifying Approach towards a Two-Valued Model Theory for Partial Algebras. Studia Sci. Math. Hungar. 16 (1981) 325–372
2. Andréka, H., Németi, I.: Generalization of Variety and Quasivariety Concepts to Partial Algebras through Category Theory. Dissertationes Mathematicæ (Rozprawy Mathematyczne) 204 (Warsaw, 1983)
3. Astesiano E., Cerioli, M.: On the Existence of Initial Models for Partial (Higher-Order) Conditional Specifications, In: Diaz, J., Orejas, F. (Eds.) TAPSOFT'89, LNCS 351 (Springer-Verlag, Berlin, 1989) 74–88
4. Astesiano E., Cerioli, M.: Commuting between Institutions via Simulation. University of Genova, Formal Methods Group, Technical Report n. 2 (1990)
5. Barwise, K. J.: Axioms for Abstract Model Theory. Ann. Math. Logic 7 (1974) 221–265