// onPercentage: voidfunc(Number)
static <A> ItIt<A> iterateOverListSendingPercentage(O onPercentage, L<A> l) {
  ret iff(new O {
    int n = l(l), i = 0, lastPercent = 0;
    
    public O get() {
      if (i >= n) ret endMarker();
      O o = l.get(i++);
      int percent = intPercentRatio(i, n);
      if (percent > lastPercent)
        pcallF(onPercentage, lastPercent = percent);
      ret o;
    }
  });
}