code: public  boolean addAll(Collection<? extends E> c) {
          // Use linear-time version if applicable
          if (m.size()==0 && c.size() > 0 &&
              c instanceof SortedSet &&
              m instanceof TreeMap) {
              SortedSet<? extends E> set = (SortedSet<? extends E>) c;
              TreeMap<E,Object> map = (TreeMap<E, Object>) m;
              Comparator<?> cc = set.comparator();
              Comparator<? super E> mc = map.comparator();
              if (cc==mc || (cc != null && cc.equals(mc))) {
                  map.addAllForTreeSet(set, PRESENT);
                  return true;
              }
          }
          return super.addAll(c);
      }
docs: Adds all of the elements in the specified collection to this set.
  
  @param c collection containing elements to be added to this set
  @return {@code true} if this set changed as a result of the call
  @throws ClassCastException if the elements provided cannot be compared
  with the elements currently in the set
  @throws NullPointerException if the specified collection is null or
  if any element is null and this set uses natural ordering, or
  its comparator does not permit null elements