sclass RSTVar<A> extends VarWithNotify<A> {
  IVF1<A> action;
  A lastSentValue;
  ReliableSingleThread rst = new(l0 update);
  
  *(IVF1<A> *action) {
    onChange(rst);
  }
  
  private void update {
    A value = get();
    if (!eq(lastSentValue, value)) {
      lastSentValue = value;
      action?.get(value);
    }
  }
}