static L<S> dropPunctuationExcept(L<S> tok, L<S> keep) {
  tok = new ArrayList<S>(tok);
  for (int i = 1; i < tok.size(); i += 2) {
    S t = tok.get(i);
    if (t.length() == 1 && !Character.isLetter(t.charAt(0)) && !Character.isDigit(t.charAt(0)) && !keep.contains(t)) {
      tok.set(i-1, tok.get(i-1) + tok.get(i+1));
      tok.remove(i);
      tok.remove(i);
      i -= 2;
    }
  }
  return tok;
}

static S dropPunctuationExcept(S s, L<S> keep) {
  ret join(dropPunctuationExcept(nlTok(s), keep));
}