7. The method according to claim 1 , wherein specifying the one or more enabling relationships comprises specifying a plurality of enabling relationships by: specifying a hierarchical attribute having a plurality of values and a refinement relation over the values; and transforming the hierarchical attribute into the plurality of enabling relationships in accordance with the refinement relation.
8. The method according to claim 7 , wherein the a plurality of values of the hierarchical relationship are denoted by w n , wherein the refinement relation is denoted by w i <h w j , and wherein transforming the hierarchical attribute into the plurality of enabling relationships comprises: creating respective attributes awi for values w i of the hierarchical attribute; assigning, to each of the attributes awi, a value w j that immediately refines w i , to create respective attribute-value pairs awi:wj; and for each of the attribute-value pairs awi:wj, creating an enabling relationship comprising the attribute-value pair awi:wj and an enableable attribute awj.