static L<S> dropCertainPunctuationAtEnd(L<S> tok, S charsToDrop) {
  tok = cloneList(tok);
  for (int i = tok.size()-2; i > 0; i -= 2) {
    S t = tok.get(i);
    if (t.length() == 1 && charsToDrop.contains(t)) {
      tok.remove(i+1);
      tok.remove(i);
    } else break;
  }
  return tok;
}

static S dropCertainPunctuationAtEnd(S s, S chars) {
  ret join(dropCertainPunctuationAtEnd(nlTok(s), chars));
}