6. The method of claim 3 , further comprising: for at least one of said substitutable terms in a given representative query, calculating at least one second substitutability metric, each second substitutability metric measuring the appropriateness of substituting that substitutable term with a substitute expression of said plurality of substitute expressions.
6. The method of claim 3 , further comprising: for at least one of said substitutable terms in a given representative query, calculating at least one second substitutability metric, each second substitutability metric measuring the appropriateness of substituting that substitutable term with a substitute expression of said plurality of substitute expressions. 8. The method of claim 6 , further comprising: upon determining that said first substitutability metric for a particular substitute expression does not meet said first pre-defined threshold, comparing said second substitutability metric for that particular substitute expression to a second pre-defined threshold, wherein that particular substitute expression is chosen upon determining that said second substitutability metric for that particular substitute expression meets said second pre-defined threshold.