/** Abridged from java.util.Set.
    Implemented by java.util.{Hash,Tree}Set.
*/
public interface Set<E>
{
   boolean add(E item);    // return false if already in the set
   boolean contains(Object o);
   boolean isEmpty();
   boolean remove(Object o);     // return false if not found
   int size();
   Iterator<E> iterator();
}