docs: @Override
      public Properties getOutputProperties(
              HttpServletRequest request, HttpServletResponse response) {
          final Properties mergedProperties = new Properties();
  
          for (final TransformerConfigurationSource source : this.sources) {
              final Properties properties = source.getOutputProperties(request, response);
              if (properties != null) {
                  mergedProperties.putAll(properties);
              }
          }
  
          return mergedProperties;
      }