1. A computer-implemented method of indexing a plurality of responses for later providing to users in response to queries, said method comprising: storing said plurality of responses; for each of said responses, storing a plurality of representative queries in association with that response, each of said representative queries representing a possible query for searching for information addressed by that response; processing each of said representative queries to determine, for each representative query, corresponding substitute expressions for particular substitutable terms in that representative query from a plurality of substitute expressions, wherein a substitute expression is chosen for a particular one of said substitutable terms by calculating at least one first substitutability metric for that one of said substitutable terms and that substitute expression based on a measure of times that substitutable term was not substituted by that substitute expression in already processed ones of said representative queries and a measure of times that substitutable term was substituted by that substitute expression in already processed ones of said representative queries, at least one of said substitutions of that substitutable term by a substitute expression in one of said already processed ones of said representative queries having been received as input from an operator; selectively modifying said representative queries by substituting substitutable terms of said representative queries by corresponding substitute expressions having a suitable first substitutability metric; for each of said plurality of responses: forming a Boolean expression from those representative queries, as selectively modified, of said plurality of representative queries associated with that response, said Boolean expression satisfied by each of those representative queries; and storing said Boolean expression in association with that response for later applying to each of said queries, and that response to be provided to said user in response of those of said queries satisfying said Boolean expression.