Weiming Liu Sanjiang Li Jochen Renz Combining RCC-8 with Qualitative Direction Calculi: Algorithms and Complexity. 854-859 2009 IJCAI http://ijcai.org/papers09/Papers/IJCAI09-146.pdf conf/ijcai/2009 db/conf/ijcai/ijcai2009.html#LiuLR09