static IterableIterator<S> ngrams_iterator(final int n, fS s) {
  final int max = l(s)-n;
  ret iteratorFromFunction(new F0<S> {
    int i;
    
    S get() {
      if (i > max) null;
      S x = substring(s, i, i+n);
      ++i;
      ret x;
    }
  });
}