docs: @Override
      public String toXML(org.jivesoftware.smack.packet.XmlEnvironment enclosingNamespace) {
          StringBuilder buf = new StringBuilder();
          if (message != null) {
              buf.append("<error type=\"cancel\">");
              buf.append('<').append(message).append(" xmlns=\"").append(NAMESPACE).append(
                      "\"/>");
              buf.append("</error>");
          }
          return buf.toString();
      }