8. The storage medium of claim 7 , wherein the content hierarchy comprises a directed acyclic graph, specifying one or more assignments of relative prominence for the content items of the content hierarchy, and one or more assignments of relative distance for ordered pairs of content items of the content hierarchy; wherein the content item N is a first content item, and the content hierarchy further includes a second content item (N 2 ) and a third content item (N 3 ), where none of N, N 2 , and N 3 is an ancestor of any other of N, N 2 , and N 3 according to the directed acyclic graph, where the prominence of N 2 is greater than or equal to the prominence of N 3 , where the distance from N 3 to N is greater than or equal to the distance from N 2 to N; and wherein determine comprises determine a topical relevance score of N for the concept that is greater, when N 3 has a greater topical relevance score determined for the concept than N 2 , than when N 2 has a greater topical relevance score determined for the concept than N 3 .
8. The storage medium of claim 7 , wherein the content hierarchy comprises a directed acyclic graph, specifying one or more assignments of relative prominence for the content items of the content hierarchy, and one or more assignments of relative distance for ordered pairs of content items of the content hierarchy; wherein the content item N is a first content item, and the content hierarchy further includes a second content item (N 2 ) and a third content item (N 3 ), where none of N, N 2 , and N 3 is an ancestor of any other of N, N 2 , and N 3 according to the directed acyclic graph, where the prominence of N 2 is greater than or equal to the prominence of N 3 , where the distance from N 3 to N is greater than or equal to the distance from N 2 to N; and wherein determine comprises determine a topical relevance score of N for the concept that is greater, when N 3 has a greater topical relevance score determined for the concept than N 2 , than when N 2 has a greater topical relevance score determined for the concept than N 3 . 10. The storage medium of claim 8 , wherein determine comprises: when the concept is a result of applying a juxtaposition operator O to a first sub-concept and a second sub-concept, a second content hierarchy C 2 includes a second content item N 2 , a first sub-hierarchy of C whose root in N is identical to a second sub-hierarchy of C 2 whose root in N 2 , with respect to member content items, parent-child relationships, assignments of relative prominence to content items, assignments of distances between content items, and assignments of topical relevance scores for the concept, the first sub-concept, and the second sub-concept to member content items other than N and N 2 , a topical relevance score for the first sub-concept for N 2 in C 2 is greater than or equal to the topical relevance score for the first sub-concept for N in C, and a topical relevance score for the second sub-concept for N 2 in C 2 is greater than or equal to the topical relevance score for the second sub-concept for N in C, determine comprises determine a topical relevance score for the concept for N in C that is greater than a topical relevance score for the concept for N 2 in C 2 .