static S ai_nlVary(S s) {
  LL<S> tok = ai_nlClassify1(s); // last element in each place is quoted original
  for (int i = 1; i < l(tok); i += 2) {
    L<S> l = tok.get(i);
    S word = ai_varyClassification(l);
    if (word != null)
      tok.set(i, ll(quoteUnlessQuoted(word)));
  }
  ret join(map(func(L<S> set) { unquote(last(set)) }, tok));
}