A. de Moor. Proceedings of the 12th International Conference on Conceptual Structures (ICCS 2004), volume 3127 of Lecture Notes in Computer Science, page 261-274. Springer, (2004)
M. Keeler, L. Searle, and C. Kloesel. Proceedings of the Fifth International Conference on
Conceptual Structures, University of Washington, Seattle, August 3--8,
1997, page 505--520. (1997)