static <A> Set<A> setWithNotifyOnAdd(final VF1<A> onAdd) {
  ret setWithNotifyOnAdd(onAdd, new HashSet);
}

static <A> Set<A> setWithNotifyOnAdd(final VF1<A> onAdd, Set<A> set) {
  ret new NotifyingSet<A>(set) {
    void change() {}
    
    public bool add(A a) {
      if (!super.add(a)) false;
      ret true with callF(onAdd, a);
    }
    
    public bool addAll(Collection<? extends A> l) {
      bool change = false;
      for (A a : l) if (add(a)) change = true;
      ret change;
    }
  };
}