code: private Optional<String> getNonRDFSourceDescribedByUri(final String resourceUri) throws IOException {
          Optional<String> nonRdfSourceDescUri = Optional.empty();
          final Header[] links = getLinkHeaders(resourceUri);
          if ( links != null ) {
              String descriptionUri = null;
              boolean isNonRDFSource = false;
              for ( final Header h : links ) {
                  final FcrepoLink link = new FcrepoLink(h.getValue());
                  if ( link.getRel().equals("describedby") ) {
                      descriptionUri = link.getUri().toString();
                  } else if ( link.getUri().toString().contains(NON_RDF_SOURCE_URI)) {
                      isNonRDFSource = true;
                  }
              }
              LOGGER.debug("isNonRDFSource: " + isNonRDFSource);
              if (isNonRDFSource && descriptionUri != null) {
                  nonRdfSourceDescUri = Optional.of(descriptionUri);
              }
          }
          return nonRdfSourceDescUri;
      }
docs: /*
  Get the describedBy Uri if the resource has a NON_RDF_SOURCE_URI link header.