unionVarSet, unionVarSets,
intersectVarSet, intersectsVarSet, disjointVarSet,
isEmptyVarSet, delVarSet, delVarSetList, delVarSetByKey,
unionVarSet, unionVarSets,
intersectVarSet, intersectsVarSet, disjointVarSet,
isEmptyVarSet, delVarSet, delVarSetList, delVarSetByKey,