8. An apparatus, comprising: a memory; and a processor configured: to receive a first query term from a user, to submit, to a search engine, a first query comprising the first query term, to receive first query term; receiving, in response to submitting the first query, a first list comprising first search results having respective first ranking, to derive multiple keywords from the first query term, to submit, for each given derived keyword, a respective second query to the search engine, the respective second query comprising the first query term and the given derived keyword, to receive, in response to submitting each of the respective second queries, a respective second list comprising respective second search results having respective second rankings, to compute, for each given first search result appearing in one or more of the second lists, a stability score that computes a stability of the given first search result across the second queries by measuring a difference between the first ranking of the given first search result from the first query in the first list and the second rankings of the given first search result in the second lists, to re-rank the first search results appearing in one or more of the second list based on their respective first rankings and stability scores in a manner preferring first search results having stability scores indicative that their ranking in the second lists is greater than or equal their ranking in the first list, and to present the re-ranked first search results to the user.
12. The apparatus according to claim 8 , wherein the processor is configured to compute a first list quality score for the first list, and to compute respective second list quality scores for each of the second lists, and wherein the processor is configured to compute the one or more additional stability scores for a given second search result in a given second list if the second list quality score for the given second list is greater than the first list quality score.