1. A computer-implemented method, 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.