code: public static <E extends Enum<E>> StreamableEnumSet<E> complementOf (StreamableEnumSet<E> s)
      {
          Class<E> elementType = s._elementType;
          StreamableEnumSet<E> set = new StreamableEnumSet<E>(elementType);
          for (E constant : elementType.getEnumConstants()) {
              if (!s.contains(constant)) {
                  set.add(constant);
              }
          }
          return set;
      }
docs: Creates a set containing all elements <em>not</em> in the set provided.