docs: public static Set<String> getFixedURLs(final ContentSpec contentSpec) {
          final Set<String> fixedUrls = new HashSet<String>();
  
          for (final Node childNode : contentSpec.getNodes()) {
              if (childNode instanceof SpecNode) {
                  final SpecNode specNode = ((SpecNode) childNode);
                  if (!isNullOrEmpty(specNode.getFixedUrl())) {
                      fixedUrls.add(specNode.getFixedUrl());
                  }
              }
  
              if (childNode instanceof Level) {
                  fixedUrls.addAll(getFixedURLs((Level) childNode));
              }
          }
  
          fixedUrls.addAll(getFixedURLs(contentSpec.getBaseLevel()));
  
          return fixedUrls;
      }