J. Hidders, J. Paredaens, R. Vercammen, and S. Demeyer. Proceedings of Second International XML Database Symposium, volume 3186 of Lecture Notes in Computer Science, page 5-20. Toronto, Canada, Springer-Verlag, (August 2004)
E. Lambrichts, P. Nees, J. Paredaens, P. Peelman, and L. Tanca. MFDBS 89, 2nd Symposium on Mathematical Fundamentals of Database Systems, 364, page 301--316. Visegrád, Hungary, Springer-Verlag, New York, (June 1989)