!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"));

  assertEqualsVerbose(false, checkNTokenHygiene("c d}"));

  assertEqualsVerbose(false, checkNTokenHygiene("c d}{"));

  assertEqualsVerbose(true, checkNTokenHygiene("{c d}{"));

  assertEqualsVerbose(true, checkNTokenHygiene("}{c d}"));

  assertEqualsVerbose(false, checkNTokenHygiene("{a b"));

}

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 = 0; i < l(tok)-1; i += 2) {

    S t = tok.get(i);

    print("t=" + quote(t));

    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 checkBackward(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(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;

}