static abstract class SmartIterator {
  int n, i;
  SmartIterator bottom;
  
  O next();
}

static class SIList extends SmartIterator {
  List list;
  
  *(List *list) { n = list.size(); }
  
  O next() {
    if (i < n)
      return list.get(i++);
    return null;
  }
}

static class SIRepeat extends SmartIterator {
  SmartIterator base;
  
  *(SmartIterator *base) {}
  
  O next() {
    if (i == 0) return "";
    
  }
}