public <R extends Set<T>> R differenceInto(SetIterable<? extends T> subtrahendSet, R targetSet) {
   return SetIterables.differenceInto(this, subtrahendSet, targetSet);
 }
 public ImmutableSortedSet<T> difference(SetIterable<? extends T> subtrahendSet) {
   return SetIterables.differenceInto(
           this, subtrahendSet, TreeSortedSet.<T>newSet(this.comparator()))
       .toImmutable();
 }