docs: protected final String getTaxonomy(final Element desc) {
          String taxonomy = null;
          final Element topic = desc.getChild("topic", getTaxonomyNamespace());
          if (topic != null) {
              final Attribute resource = topic.getAttribute("resource", getRDFNamespace());
              if (resource != null) {
                  taxonomy = resource.getValue();
              }
          }
          return taxonomy;
      }