static LL<S> ai_nlVary(S s) {
  LL<S> tok = nlClassify1(s); // last element is quoted original
  for (int i = 1; i < l(tok); i += 2) {
    L<S> l = tok.get(i);
    continue if l(l) <= 1;
    S category = first(l);
    S word = random(ai_texts("$X", "is a", category));
    if (word != null)
      tok.set(i, ll(quoteUnlessQuoted(word)));
  }
  ret join(map(func(L<S> set) { unquote(last(set)) }, tok));
}