O. Balovnev, M. Breunig, and A. Cremers. Advances in Spatial Databases, volume 1262 of Lecture Notes in Computer Science, Springer, Berlin / Heidelberg, (1997)
M. Atzmueller, S. Beer, and F. Puppe. Proc. 22nd International Florida Artificial Intelligence Research Society Conference (FLAIRS), page 402--407. AAAI Press, (2009)