Hélène Fargier Nic Wilson Algebraic Structures for Bipolar Constraint-Based Reasoning. 623-634 2007 conf/ecsqaru/2007 ECSQARU http://dx.doi.org/10.1007/978-3-540-75256-1_55 db/conf/ecsqaru/ecsqaru2007.html#FargierW07