sclass SetInConcept<E> extends CollectionInConcept<E> implements Set<E> {
  *() {} // for persistence
  *(Set<E> set, Concept cc) {
    super(set, cc);
  }
  
  public boolean equals(Object o) {
      if (this == o)
          return true;
      synchronized (this) {return c.equals(o);}
  }
  
  public int hashCode() {
      synchronized (this) {return c.hashCode();}
  }
}