1. H.-J. Appelrath. Das EUREKA-Projekt PROTOS. In W. Brauel and W. Wahlster, editors, Proceedings GI-Kongreß Wissensbasierte Systeme, pages 1–11, Springer-Verlag, Informatik- Fachberichte 155, 1987.
2. S. Böttcher. The architecture of the PROTOS-L system. In H.-J. Appelrath, A. B. Cremers, and H. Schiltknecht, editors, Proc. First PROTOS Workshop, Morcote, Switzerland, 1988.
3. S. Böttcher and C. Beierle. Data base support for the PROTOS-L system. In Proceedings EUROMICRO-89, North Holland, 1989.
4. Douglas DeGroot and Gary Lindstrom, editors. Functional and Logic Programming. Prentice Hall, 1986.
5. R. Dietrich and F. Hagl. A polymorphic type system with subtypes for Prolog. In Proceedings of the 2nd European Symposium on Programming, pages 79–93, volume 300 of Lectures Notes in Computer Science, Springer-Verlag, Berlin, Heidelberg, New York, 1988.