static IterableIterator<S> sentencePrefixes_notEmpty_rev(fS sentence) {
  fL<S> tok = dropLast(javaTok(sentence));
  ret iff(new F0 {
    int i = l(tok);
    
    public O get() {
      if (i <= 0) ret endMarker();
      S s = joinSubList(tok, 0, i);
      i -= 2;
      ret s;
    }
  });
}