static IterableIterator<S> allWordsOfAlphabet_fixedLength(fS alphabet, final int length) {
  ret new IterableIterator<S>() {
    L<Int> l = rep((Int) 0, length);
    
    public bool hasNext() {
      ret true; // should change
    }
    
    public S next() {
      S s = makeString();
      int i = 0;
      while (i < l(l)) {
        int n = l.get(i);
        if (n+1 < l(alphabet)) {
          l.set(i, n+1);
          break;
        }
        l.set(i++, 0);
      }
      if (i >= l(l)) null;
      ret s;
    }
    
    S makeString() {
      StringBuilder buf = new StringBuilder(l(l));
      for (int i = 0; i < l(l); i++)
        buf.append(alphabet.charAt(l.get(i)));
      ret str(buf);
    }
  };
}