public <R extends Set<T>> R symmetricDifferenceInto(SetIterable<? extends T> set, R targetSet) {
   return SetIterables.symmetricDifferenceInto(this, set, targetSet);
 }
 public ImmutableSortedSet<T> symmetricDifference(SetIterable<? extends T> setB) {
   return SetIterables.symmetricDifferenceInto(
           this, setB, TreeSortedSet.<T>newSet(this.comparator()))
       .toImmutable();
 }