sbool checkNTokenHygiene(L<S> tok) {
  ret l(tok) == 1 || checkNTokenHygiene_forward(tok) && checkNTokenHygiene_backward(tok);
}

sbool checkNTokenHygiene_forward(L<S> tok) {
  int level = 0;
  for (int i = 0; i < l(tok)-1; i += 2) {
    S t = tok.get(i);
    for (char c : characters(t))
      if (c == '{') ++level;
      else if (c == '}') level = max(0, level-1);
  }
  if (level == 0) true;
  for (char c : characters(last(tok)))
    if (c == '}') if (--level == 0) true;
  false;
}

sbool checkNTokenHygiene_backward(L<S> tok) {
  int level = 0;
  for (int i = l(tok)-1; i > 0; i -= 2) {
    S t = tok.get(i);
    for (char c : reversedCharacters(t))
      if (c == '}') ++level;
      else if (c == '{') level = max(0, level-1);
  }
  if (level == 0) true;
  for (char c : reversedCharacters(first(tok)))
    if (c == '{') if (--level == 0) true;
  false;
}