// f: func -> A | endMarker
static <A> IterableIterator<A> filterIterator(final Iterable<A> it, final F1<A, Bool> f) {
  if (it == null) null;
  ret iff(func -> O {
    while (it.hasNext()) {
      A a = it.next();
      if (callF(f, a))
        ret a;
    }
    ret endMarker();
  });
}