static S ai_groupNounPhrases(S s) {
  L<S> tok = defaultTok(s);
  for (int i = 1; i < l(tok); ) {
    int j = ai_endOfNounPhrase(subList(tok, i-1)); // j is odd
    if (j <= 3) i += 2;
    else {
      tok.set(i, "{" + tok.get(i));
      int i2 = i+j-3;
      tok.set(i2, tok.get(i2) + "}");
      i = i2+2;
    }
  }
  ret join(tok);
}