1. A computer-implemented method of searching for information in an information retrieval system, the computer-implemented method comprising: receiving a search query comprising a plurality of terms; selecting one of the plurality of terms, wherein unselected terms of the plurality of terms comprise remaining terms of the received search query and the selected term has a position relative to the remaining unselected terms of the received search query; selecting, for the selected term, a replacement term that appears in a plurality of previously received multi-term search queries in a same position within the previously received multi-term search queries as the position of the selected term relative to the remaining unselected terms of the received search query; deriving an altered query by replacing the selected term in the received query with a disjunction of the selected term and the replacement term; and generating a list of search results for the altered query.
10. The computer-implemented method of claim 1 , further comprising: identifying a pair of multi-term queries in the query logs, wherein a first multi-term query of the pair includes a first phrase in a particular relative position within the first multi-term query and a second multi-term query of the pair includes a second phrase in the particular relative position within the second multi-term query; determining a number of search results that are common to the pair of multi-term queries; and identifying the first phrase as synonymous with the second phrase in the context of the received multi-term query in response to the number of search results that are common to the pair of multi-term queries exceeding a threshold.