!7

p-exp {
  assertEqualsVerbose(true, checkNTokenHygiene("{a b} {c d}"));
  assertEqualsVerbose(false, checkNTokenHygiene("a b} {c d}"));
  assertEqualsVerbose(false, checkNTokenHygiene("{a b} {c d"));
  assertEqualsVerbose(true, checkNTokenHygiene("a b c d"));
}

sbool checkNTokenHygiene(S s) {
  L<S> tok = javaTokNPunctuation(s);
  if (l(tok) == 1) true;
  ret print("checkForward " + sfu(tok) + ": ", checkForward(tok))
    && print("checkBackward " + sfu(tok) + ": ", checkBackward(tok));
}

sbool checkForward(L<S> tok) {
  int level = 0;
  for (int i = 2; i < l(tok)-1; i += 2)
    for (char c : characters(tok.get(i)))
      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 checkBackward(L<S> tok) {
  int level = 0;
  for (int i = l(tok)-2; i > 0; i -= 2)
    for (char c : reversedCharacters(print("tok", tok.get(i))))
      if (c == '}') ++level;
      else if (c == '{') level = max(0, level-1);
  print("level at beginning: " + level);
  if (level == 0) true;
  for (char c : reversedCharacters(first(tok)))
    if (c == '{') if (--level == 0) true;
  false;
}