// order of returned elements is undefined, but root is returned first
// doesn't uniquify output, so make sure you don't produce a loop 
static <A> ItIt<A> recursiveIterator(A root, IF1<A, Iterable<A>> getChildren) {
  LinkedList<A> list = llLinkedList(root);
  ret iff(func {
    if (empty(list))
      ret endMarker();
    A a = popFirst(list);
    addAll(list, getChildren.get(a));
    ret a;
  });
}