docs: public XMLAttributeList addAttribute(@Nonnull String name, @Nonnull String value) {
          if (name == null) {
              throw new NullPointerException("name must be nonnull");
          }
          if (value == null) {
              throw new NullPointerException("value must be nonnull");
          }
          nameValuePairList.add(new NameValuePair(name, value));
          return this;
      }