static <A> ItIt<A> iterateUpdatingProgressShower(IProgressShower progressBar, Collection<A> l) {
  if (progressBar == null) ret iterableIterator(l);
  ret iteratorFromFunction_withEndMarker_f0(new F0 {
    int n = l(l), i;
    Iterator<A> it = iterator(l);
    NotTooOften nto = onlyTenTimesASecond();
    
    { progressBar.setTotal(n); }
    
    void update {
      progressBar.setValue(i);
      progressBar.setText(i + " / " + n);
    }
    
    public O get() {
      if (!it.hasNext()) {
        i = n;
        update();
        ret endMarker();
      }
      if (nto.yo()) update();
      ++i;
      ret it.next();
    }
  });
}