1. In a computerized search system which queries are submitted by users who receive, in response, a list of documents selected from a corpus of documents wherein the list comprises documents deemed responsive to a user's query, a method of preprocessing the query comprising: obtaining a base query from a user, wherein the base query comprises a plurality of words; determining a base distribution of nodes of a taxonomy that have non-zero probabilities of being relevant to the base query, wherein the taxonomy is a taxonomy of topics into which documents of the corpus of documents might be assigned; modifying the base query to form a truncated query when it is determined that the base query will return no results, wherein modifying the base query to form the truncated query comprises: identifying word pairs in the base query, determining pair distributions for word pairs over the taxonomy, selecting a desired word pair based in part on the pair distributions, generating a first query by omitting from the base query a first word of the desired word pair, generating a second query by omitting from the base query a second word from the desired word pair, determining a first count of documents corresponding to the first query, determining a second count of documents corresponding to the second query, and determining at least one word to remove from the base query based on the first and second counts such that the truncated query comprises a portion of the base query from which the at least one word is removed; running the truncated query against the corpus of documents to obtain a results list of one or more documents in the document corpus deemed responsive to the truncated query; and outputting the results list as the list comprising documents deemed responsive to the user's query.