Relation Variables in Qualitative Spatial Reasoning
  We study an alternative to the prevailing approach to modelling qualitative spatial reasoning (QSR) problems as constraint satisfaction problems. In the standard approach, a relation between objects is a constraint whereas in the alternative approach it is a variable. The relation-variable approach greatly simplifies integration and implementation of QSR. To substantiate this point, we discuss several QSR algorithms from the literature which in the relation-variable approach reduce to the customary constraint propagation algorithm enforcing generalised arc-consistency. 
Sebastian Brand