static <A> ItIt<A> itIt(ItIt<A> it) {
  if (it == null) ret emptyItIt();
  ret it;
}

static <A> ItIt<A> itIt(Iterator<A> it) {
  if (it == null) ret emptyItIt();
  if (it cast ItIt) ret it;
  ret new IterableIterator<A> {
    public bool hasNext() { ret it.hasNext(); }
    public A next() { ret it.next(); }
  };
}

static <A> ItIt<A> itIt(Iterable<A> l) {
  if (l == null) ret emptyItIt();
  if (l cast ItIt) ret l;
  Iterator<A> it = l.iterator();
  if (it cast ItIt) ret it;
  ret new IterableIterator<A> {
    public bool hasNext() { ret it.hasNext(); }
    public A next() { ret it.next(); }
  };
}