42. The system of claim 36 , the operations further comprising: 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.
42. The system of claim 36 , the operations further comprising: 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. 43. The system of claim 42 , wherein the search query is textually distinct from each of the selected sibling queries and all the substrings of the selected sibling queries.