docs: @SuppressWarnings("unchecked")
      public static void addIQProvider(String elementName, String namespace,
              Object provider) {
          validate(elementName, namespace);
          // First remove existing providers
          String key = removeIQProvider(elementName, namespace);
          if (provider instanceof IQProvider) {
              iqProviders.put(key, (IQProvider<IQ>) provider);
          } else {
              throw new IllegalArgumentException("Provider must be an IQProvider");
          }
      }