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