31. The computer program product of claim 23 , wherein at least one of the indexes is an attribute index of a concept referring to a set of multiple concepts within the same category forming a parent-child relationship, the index comprising computing transitive closure along a specified direction in the parent-child relationship for indexing all reachable concepts moving in the specified direction given a specific concept in the category.
31. The computer program product of claim 23 , wherein at least one of the indexes is an attribute index of a concept referring to a set of multiple concepts within the same category forming a parent-child relationship, the index comprising computing transitive closure along a specified direction in the parent-child relationship for indexing all reachable concepts moving in the specified direction given a specific concept in the category. 32. The computer program product of claim 31 , wherein the parent-child relationship is a hierarchy including a single parent-multiple child relationship.