docs: public static URI setElementID(final URI relativePath, final String id) {
          String topic = getTopicID(relativePath);
          if (topic != null) {
              return setFragment(relativePath, topic + (id != null ? SLASH + id : ""));
          } else if (id == null) {
              return stripFragment(relativePath);
          } else {
              throw new IllegalArgumentException(relativePath.toString());
          }
      }