// f : A -> Collection<A>
static <A> ItIt<A> mapLike repeatFunctionReturningList_iterator(O f, A input, O... _) {
  final Set<A> seen = optParOr seen(_, () -> (Set) new HashSet);
  final Int max = cast optPar max(_);
  bool plusInput = boolPar plusInput(_); // return original input too?
  
  // Make pool of iterators
  final new LinkedList<Iterator<A>> pool;
  if (plusInput) pool.add(singletonIterator(input));
  else seen.add(input);
  pool.add(iterator((Iterable<A>) callF(f, input)));
  ret iteratorFromFunction(new F0<A> {
    int n = 0;
    
    public A get() {
      if (max != null && n >= max) null;
      while ping (!empty(pool)) {
        Iterator<A> it = first(pool);
        if (!it.hasNext()) continue with removeFirst(pool);
        
        // Get entry and check if seen already
        A 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));
        ++n;
        ret entry;
      }
      null;
    }
  });
}

Began life as a copy of #1018118