A. Srinivasan, R. King, S. Muggleton, and M. Sternberg. Proceedings of the 7th International Workshop on Inductive Logic Programming, 1297, page 273--287. Springer-Verlag, (1997)
N. Guarino, M. Carrara, and P. Giaretta. KR'94: Principles of Knowledge Representation and Reasoning, Morgan Kaufmann, San Francisco, California, (1994)