19. The method of claim 13 , wherein the search expression comprises sub-expressions E 1 , . . . , E m , and wherein for portions of the content in a substructure at a level (N 1 , . . . , N k ), iterating over one or more portions of the content at the level to capture potential of a portion to influence other portions of the level comprises iterating over N 1 , . . . , N k to compute Σ 1≦i≦k (A i ) and Σ 1≦i≦k (A i *p i ), and iterating over E 1 , . . . , E m for each N i to compute, for each 1≦i≦k and 1≦j≦m, Σ 1≦i≦k (A i *D i ), and Σ 1≦i≦k (A i *D i,j *p i ), wherein A i is relevance size of N i , D i,j is the density of N i for search expression E j , and p i is the position of N i .
19. The method of claim 13 , wherein the search expression comprises sub-expressions E 1 , . . . , E m , and wherein for portions of the content in a substructure at a level (N 1 , . . . , N k ), iterating over one or more portions of the content at the level to capture potential of a portion to influence other portions of the level comprises iterating over N 1 , . . . , N k to compute Σ 1≦i≦k (A i ) and Σ 1≦i≦k (A i *p i ), and iterating over E 1 , . . . , E m for each N i to compute, for each 1≦i≦k and 1≦j≦m, Σ 1≦i≦k (A i *D i ), and Σ 1≦i≦k (A i *D i,j *p i ), wherein A i is relevance size of N i , D i,j is the density of N i for search expression E j , and p i is the position of N i . 21. The method of claim 19 , wherein the method further comprises calculating a virtual sibling content item that corresponds to a maximum combined capacity of N 1 , . . . , N k to influence relevance to any search term, such that the position of is Σ 1≦i≦k (A i *p i )/Σ 1≦i≦k (A i ), and such that the influence capacity of , corresponding to a product of relevance size with maximum density, is Σ 1≦i≦k (A i ).