docs: public TreeRule getRule(final int level, final int order) {
      if (rules == null || rules.isEmpty()) { 
        return null;
      }
      
      TreeMap<Integer, TreeRule> rule_level = rules.get(level);
      if (rule_level == null || rule_level.isEmpty()) {
        return null;
      }
      
      return rule_level.get(order);
    }