23. The apparatus according to claim 17 , wherein the processor is configured to: generate the refinement options by generating: (a) primary refinement options that are most closely related to the search query, and (b) for at least a portion of the primary refinement options, secondary refinement options, which are more distantly related to the search query, and are also related to their associated primary refinement option, and present the primary refinement options, and the secondary refinement options for the at least the portion of the primary refinement options.
17. Apparatus comprising: an interface; and a processor, which is configured to construct an association graph that includes one or more terms as vertices linked by edges, which edges have respective edge scores that represent respective strengths of association of the respective vertices linked thereby; receive from a user, via the interface, a search query that comprises one or more of the terms of the association graph; responsively to the search query, generate, using the association graph, refinement options that comprise respective terms of the association graph, and present, via the interface, information to the user, including refinement options for optional addition to the search query; identify a portion of the refinement options, and corresponding terms, that the user did not select; using the association graph, assign respective ranking scores to search results generated responsively to the search query, including lowering one or more of the ranking scores of the respective search results that are characterized by the non-selected portion of the refinement options, by damping the edge score of an edge of the association graph between two of the vertices respectively representing two of the non-selected terms; rank the search results according to the ranking scores; and present the ranked search results to the user, via the interface.