1. A method for processing stored data, comprising: storing a collection of data units, said data units comprising documents; retrieving at least one data unit in response to a query, the query comprising a search operator comprised of a plurality of search sub-expressions each having an associated weight value; and outputting as a result to the query the retrieved at least one data unit, where said retrieved at least one data unit has a weight value sum that exceeds a threshold weight value sum, where said search operator comprises a weighted AND function, where varying the threshold weight value sum varies the operation of the weighted AND function from being substantially a logical OR function to being substantially a logical AND function, where at least one of the weight value sum and the threshold weight value sum is variable during a search.