docs: public final void addExtraTag(final String name, final String value) {
      if (name.length() <= 0) {
        throw new IllegalArgumentException("empty tag name, value=" + value);
      } else if (value.length() <= 0) {
        throw new IllegalArgumentException("empty value, tag name=" + name);
      } else if (name.indexOf('=') != -1) {
        throw new IllegalArgumentException("tag name contains `=': " + name
                                           + " (value = " + value + ')');
      } else if (value.indexOf('=') != -1) {
        throw new IllegalArgumentException("tag value contains `=': " + value
                                           + " (name = " + name + ')');
      }
      if (extratags == null) {
        extratags = new HashMap<String, String>();
      }
      extratags.put(name, value);
    }