static ItIt<S> repeatedInputModificationsIterator(S input, F1<S, L<Iterator<S>>> makeIterators) {
  final new HashSet<S> seen;
  
  // Make pool of iterators
  final new LinkedList<Iterator<S>> pool;
  addAll(pool, callF(makeIterators, input));
  ret iteratorFromFunction(func -> S {
    while ping (!empty(pool)) {
      Iterator<S> it = first(pool);
      if (!it.hasNext()) continue with removeFirst(pool);
      
      // Get entry and check if seen already
      S entry = it.next();
      if (!seen.add(entry)) continue;

      // found new entry - return and schedule for further analysis
      addAll(pool, callF(makeIterators, input));
      ret entry;
    }
    null;
  });
}