Amar Isli Anthony G. Cohn An Algebra for Cyclic Ordering of 2D Orientations. 643-649 1998 conf/aaai/1998 AAAI/IAAI db/conf/aaai/aaai98.html#IsliC98 http://www.aaai.org/Library/AAAI/1998/aaai98-091.php