8. The method according to claim 7 , wherein the user is one of a plurality of users, and wherein generating the first set of refinement terms comprises generating the first set of refinement terms by: constructing a topic profile that represents interactions of a plurality of previous searches conducted by the plurality of users within a single topic, wherein each of the previous searches includes one or more previous query terms, and selecting at least one of the previous query terms for inclusion in the first set as one of the refinement terms, responsively to a level of association of the previous query term with the first query terms in the topic profile.
8. The method according to claim 7 , wherein the user is one of a plurality of users, and wherein generating the first set of refinement terms comprises generating the first set of refinement terms by: constructing a topic profile that represents interactions of a plurality of previous searches conducted by the plurality of users within a single topic, wherein each of the previous searches includes one or more previous query terms, and selecting at least one of the previous query terms for inclusion in the first set as one of the refinement terms, responsively to a level of association of the previous query term with the first query terms in the topic profile. 9. The method according to claim 8 , wherein the topic profile includes a topic association graph (TAG), wherein constructing the personal profile comprises constructing the TAG including at least a portion of the previous query terms as vertices, and wherein selecting the previous query term comprises selecting the previous query term for inclusion in the first set responsively to finding that the previous query term is directly linked to all of the first query terms in the TAG.