1. A computer-implemented method comprising: receiving an indication that a synonym rule that was previously determined, at a first time, to be included in a collection of synonym rules is determined not to be included in the collection at a second time that is later than the first time, wherein, at the second time, criteria used to determine which synonym rules are to be included in the collection are different from criteria used at the first time to determine which synonym rules are to be included in the collection; receiving historic usage data relating to the synonym rule; determining, using the historic usage data relating to the synonym rule, to add the synonym rule to the collection despite receiving the indication that the synonym rule is determined not to be included in the collection; and in response to determining to add the synonym rule, adding the synonym rule to the collection.