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; selectively modifying said representative queries by substituting terms of said representative queries by corresponding substitute expressions chosen from a plurality of substitute expression, wherein a substitute expression is chosen for a particular term in one of said representative queries based on past substitutions in others of said representative queries; 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.