static <A> ItIt<A> uniqueRecursiveIterator(Cl<A> rootElements, IF1<A, Iterable<A>> getChildren) {
  new Set<A> set;
  addAll(set, rootElements);
  LinkedList<A> queue = cloneAsLinkedList(set);
  ret iff(func {
    if (empty(queue))
      ret endMarker();
    A a = popFirst(queue);
    addAllWithTesterSet(queue, set, getChildren.get(a));
    ret a;
  });
}