1. A method of processing a stabbing query comprising: obtaining a signal representative of said stabbing query; identifying nodes in a tree, said tree being stored in a memory, each of a plurality of sets being associated with one or more nodes of said tree, which are candidate nodes for being associated with at least one set which is stabbed by said stabbing query, each of sets being associated with a score; and for at least one of said candidate nodes, determining whether at least one unselected one of said sets associated with said candidate node is stabbed by said stabbing query and, if said at least one of said unselected sets associated said candidate node is stabbed by said stabbing query, determining a highest scoring unselected set associated with said candidate node which is stabbed by said stabbing query.