sclass SynchronizedSet<A> extends SynchronizedCollection<A> implements Set<A> {
  *() {} // for persistence
  *(Set<A> set) { super(set); }
  *(Set<A> set, O mutex) { super(set, mutex); }
  
  public boolean equals(Object o) {
      if (this == o)
          return true;
      synchronized (this) {return c.equals(o);}
  }
  
  public int hashCode() {
      synchronized (this) {return c.hashCode();}
  }
}