code: public static Set<String> getFixedURLs(final Level level) {
          final Set<String> fixedUrls = new HashSet<String>();
  
          for (final Node childNode : level.getChildNodes()) {
              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));
              }
          }
  
          return fixedUrls;
      }
docs: Gets all the fixed urls from a content specification level.
  
  @param level A level to get the fixed urls for
  @return A set of fixed urls used in the level.