15. A system, comprising: one or more computers configured to perform operations including: receiving a first search query including one or more search terms; providing a first set of documents responsive to the first query; determining one or more collocations from the first set of documents that include at least one of the search terms, each collocation including at least one other term that is semantically related to the at least one search term; scoring the collocations from the first set of documents, including determining a mutual information score for each collocation using a mutual information test; multiplying at least some collocation scores by frequency counts, a frequency count for a given collocation representing a number of times the collocation occurs in a document of the first set of documents; determining a subset of the first set of documents to present to a user based on the scored collocations; providing one or more user interface elements to be used by a user to include and exclude documents from the first set of documents that contain at least one specified collocation of the collocations; receiving user input specifying inclusion or exclusion of documents from the first set of documents that contain at least one of the collocations; refining the first search query to produce a second query based on the user input; and providing a second set of documents responsive to the second query, where the second set of documents includes or excludes documents containing the specified collocation.
15. A system, comprising: one or more computers configured to perform operations including: receiving a first search query including one or more search terms; providing a first set of documents responsive to the first query; determining one or more collocations from the first set of documents that include at least one of the search terms, each collocation including at least one other term that is semantically related to the at least one search term; scoring the collocations from the first set of documents, including determining a mutual information score for each collocation using a mutual information test; multiplying at least some collocation scores by frequency counts, a frequency count for a given collocation representing a number of times the collocation occurs in a document of the first set of documents; determining a subset of the first set of documents to present to a user based on the scored collocations; providing one or more user interface elements to be used by a user to include and exclude documents from the first set of documents that contain at least one specified collocation of the collocations; receiving user input specifying inclusion or exclusion of documents from the first set of documents that contain at least one of the collocations; refining the first search query to produce a second query based on the user input; and providing a second set of documents responsive to the second query, where the second set of documents includes or excludes documents containing the specified collocation. 20. The system of claim 15 , further comprising: identifying collocations that include terms that occur only once in a document of the first set of documents, so the identified collocations cannot be specified by the input.