// f : A -> Collection<A>
static <A> ItIt<A> mapLike repeatFunctionReturningList_iterator(O f, A input) {
  final HashSet<A> seen = lithashset(input);
  
  // Make pool of iterators
  final new LinkedList<Iterator<A>> pool;
  pool.add(iterator((Iterable<A>) callF(f, entry)));
  ret iteratorFromFunction(func -> A {
    while ping (!empty(pool)) {
      Iterator<A> 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
      Collection<A> newStuff = cast callF(f, entry);
      if (nempty(newStuff))
        pool.add(iterator(newStuff));
      ret entry;
    }
    null;
  });
}