Varieties generated by two-element majority algebras and their equivalences K. Denecke (1986) Beiträge zur Algebra und Geometrie = Contributions to algebra and geometry
Varieties of finite categories Alex Weiss, Denis Therien (1986) RAIRO - Theoretical Informatics and Applications - Informatique Théorique et Applications
Various remarks on quasi-universal model classes. Klaus Kaiser (1975) Archiv für mathematische Logik und Grundlagenforschung