private static void addCompatiblePair(Map<LanguageLevel, LanguageLevel> levels, LanguageLevel l1, LanguageLevel l2) {
    levels.put(l1, l2);
    levels.put(l2, l1);
  }
private static void addCompatiblePair(Map<LanguageLevel, LanguageLevel> levels, LanguageLevel l1, LanguageLevel l2)