11. A non-transitory computer storage device encoded with a computer program, the program comprising instructions that when executed by one or more computers cause the one or more computers to perform operations comprising: determining, by a query reissuer, a first estimate of quality of a first set of search results that a search engine identifies as responsive to a user search query received from a user device; classifying, by the query reissuer, the first set of search results that the search engine identifies as responsive to the user search query as unacceptable based at least on the first estimate of quality of the first set of search results; and in response to classifying the first set of search results that the search engine identifies as responsive to the user search query as unacceptable based at least on the first estimate of quality of the first set of search results, transmitting, by the query reissuer, an instruction to a query reviser to generate a modified search query for submission to the search engine.
11. A non-transitory computer storage device encoded with a computer program, the program comprising instructions that when executed by one or more computers cause the one or more computers to perform operations comprising: determining, by a query reissuer, a first estimate of quality of a first set of search results that a search engine identifies as responsive to a user search query received from a user device; classifying, by the query reissuer, the first set of search results that the search engine identifies as responsive to the user search query as unacceptable based at least on the first estimate of quality of the first set of search results; and in response to classifying the first set of search results that the search engine identifies as responsive to the user search query as unacceptable based at least on the first estimate of quality of the first set of search results, transmitting, by the query reissuer, an instruction to a query reviser to generate a modified search query for submission to the search engine. 12. The device of claim 11 , wherein the operations comprise: determining, by the query reissuer, a second estimate of quality of the second set of search results that the search engine identifies as responsive to the modified search query; classifying, by the query reissuer, the second set of search results that the search engine identifies as responsive to the modified search query as acceptable based at least on the second estimate of quality of the second set of search results; and in response to classifying the second set of search results that the search engine identifies as responsive to the modified search query as acceptable based at least on the second estimate of quality of the second set of search results, selecting, by the query reissuer, a subset of the second set of search results.