docs: public static void parse(final HashMap<String, String> tags,
                             final String tag) {
      final String[] kv = splitString(tag, '=');
      if (kv.length != 2 || kv[0].length() <= 0 || kv[1].length() <= 0) {
        throw new IllegalArgumentException("invalid tag: " + tag);
      }
      if (kv[1].equals(tags.get(kv[0]))) {
          return;
      }
      if (tags.get(kv[0]) != null) {
        throw new IllegalArgumentException("duplicate tag: " + tag
                                           + ", tags=" + tags);
      }
      tags.put(kv[0], kv[1]);
    }