5. The method of claim 4 , wherein measuring the second mutual information values includes determining numbers of clicks by the users on: the shared URLs when returned by the search engine responsive to the entire second query, other second URLs that are returned by the search engine responsive to other second queries that include the second phrases without the second tags, and further second URLs that are returned by the search engine responsive to further second queries that include the second tags without the second phrases.
4. The method of claim 1 , wherein measuring the first mutual information values includes determining numbers of clicks by the users on: shared URLs that are returned by the search engine responsive to the entire first query, other first URLs that are returned by the search engine responsive to other first queries that include the first phrases without the first tags, and further first URLs that are returned by the search engine responsive to further first queries that include the first tags without the first phrases.