subVarSet       :: VarSet -> VarSet -> Bool     -- True if first arg is subset of second
        -- (s1 `intersectsVarSet` s2) doesn't compute s2 if s1 is empty;
        -- ditto disjointVarSet, subVarSet

unionVarSet     = unionUniqSets
subVarSet       :: VarSet -> VarSet -> Bool
unionVarSet     = unionUniqSets
unionVarSet     = unionUniqSets