9. The system of claim 8 , comprising: determining, by the client device, that a particular resource is hosted by the search engine; transmitting, by the client device and to the search engine, a request for the particular resource; and receiving, by the client device and from the search engine, the occurrence count of the particular resource.
8. A system comprising: one or more computers and one or more storage devices storing instructions that are operable, when executed by the one or more computers, to cause the one or more computers to perform operations comprising: transmitting, by a client device, a search query to a search engine; receiving, by the client device and from the search engine, data identifying resources that are responsive to the search query; obtaining a subset of the identified resources; determining, by the client device, a number of times that each term of a set of terms that occur in the obtained resources occurs in the obtained resources; transmitting, by the client device and to the search engine, an occurrence count that specifies the number of times that each term of the set of terms occurs in the obtained resources; and receiving, by the client device and from the search engine, code that, when invoked by the client device, implements controls for automatically reformulating the search query to include or exclude one or more frequently occurring terms identified in the occurrence count.