31. The system of claim 24 , wherein selecting one or more candidate refinement queries comprises: selecting as candidate refinement queries one or more sibling queries of the search query, wherein sibling queries are queries that have one or more shared parent queries in common, and wherein each of the sibling queries was submitted during a second search session following submission of a shared parent query during the first search session.
31. The system of claim 24 , wherein selecting one or more candidate refinement queries comprises: selecting as candidate refinement queries one or more sibling queries of the search query, wherein sibling queries are queries that have one or more shared parent queries in common, and wherein each of the sibling queries was submitted during a second search session following submission of a shared parent query during the first search session. 32. The system of claim 31 , wherein the search query is textually distinct from each of the selected sibling queries and all the substrings of the selected sibling queries.