static <A> IterableIterator<A> iterate_printProgress(final Collection<A> l) {
  ret iff(new F0 {
    int n = l(l), i;
    Iterator<A> it = iterator(l);
    NotTooOften nto = onlyEverySecond();
    
    public O get() {
      if (!it.hasNext()) ret endMarker();
      if (i > 0 && nto.yo()) print("Progress: " + i + "/" + n);
      ++i;
      ret it.next();
    }
  });
}