[Haskell-cafe] Symmetric Difference for Data.Set and Data.IntSet?

Could this be implemented as a primitive? I know it can be made from union and difference, or doing toList on both of them, taking the symmetric difference, and using fromDistinctAscList on the result, but it'd probably be much faster if it could be done natively, like can be done with Data.Map.mergeWithKey. Alternatively, could we get a merge interface for Set and IntSet roughly matching Map and IntMap?