code: private static void mapAdd(Map<String, List<String>> map, String key, String val) {
      List<String> list = map.get(key);
      if (list == null) {
        list = new ArrayList<>();
        map.put(key, list);
      }
      list.add(val);
    }
docs: Adds a key and value to a Map<String, List<String>>.