19. A computer storage medium 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: receiving an original query including a first limitation that constrains a search; modifying the original query to obtain a modified query in which the first limitation that constrains the search has been omitted; obtaining, from a search engine system, first search results responsive to the modified query, wherein the first search results have an associated ranking determined by the search engine system, and wherein each of the first search results refers to a respective resource; identifying one or more common characteristics shared by two or more resources, each of the two or more resources corresponding to a different highly-ranked result of the first search results; generating a second modified query comprising the original query and a second limitation representing the one or more common characteristics, the second limitation requiring results responsive to the second modified query to reference a resource having the one or more common characteristics; obtaining second search results responsive to the second modified query, wherein each of the second search results refers to a resource having the one or more common characteristics; and providing the second search results in a response to the original query.
19. A computer storage medium 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: receiving an original query including a first limitation that constrains a search; modifying the original query to obtain a modified query in which the first limitation that constrains the search has been omitted; obtaining, from a search engine system, first search results responsive to the modified query, wherein the first search results have an associated ranking determined by the search engine system, and wherein each of the first search results refers to a respective resource; identifying one or more common characteristics shared by two or more resources, each of the two or more resources corresponding to a different highly-ranked result of the first search results; generating a second modified query comprising the original query and a second limitation representing the one or more common characteristics, the second limitation requiring results responsive to the second modified query to reference a resource having the one or more common characteristics; obtaining second search results responsive to the second modified query, wherein each of the second search results refers to a resource having the one or more common characteristics; and providing the second search results in a response to the original query. 25. The computer storage medium of claim 19 , wherein the one or more characteristics include a plurality of characteristics, and wherein generating a second modified query comprises generating the second modified query comprising the original query, the second limitation, and a third limitation, the second limitation and the third limitation representing different characteristics of the plurality of characteristics.