At times it may be desirable to filter the source list before calculating regions. An example of such a case is if the list contains many rows for the same source position (as do eg the standard XMM source-list products). There is usually not much point in constructing more than one region for the same source position, so in this case it would be desirable to filter out all the excess rows before constructing regions. This can be achieved via the parameter expression. This takes any selection expression which obeys the selectlib grammatical rules and which involves only those columns present in the source list.