/**
   * Retrieves a list of registered predicates which can be used to create content relations.
   *
   * @return String containing a list with registered predicates.
   * @throws InvalidContentException Thrown if a xml file with an ontology has invalid content
   * @throws InvalidXmlException Thrown if a xml file with an ontology is invalid rdf/xml
   * @throws SystemException e
   */
  @Override
  public String retrieveRegisteredPredicates()
      throws InvalidContentException, InvalidXmlException, SystemException {

    final List<String> predicates = ContentRelationsUtility.getPredicates();
    final Iterator<String> it = predicates.iterator();

    final StringBuilder sb = new StringBuilder();
    sb.append(Constants.XML_HEADER);
    sb.append("<registeredPredicates:predicates");
    sb.append(
        " xmlns:registeredPredicates=\"http://www.escidoc.org/schemas/predicate-list/0.1\" >\n");
    while (it.hasNext()) {
      sb.append("<registeredPredicates:predicate>");
      sb.append(it.next());
      sb.append("</registeredPredicates:predicate>\n");
    }
    sb.append("</registeredPredicates:predicates>\n");
    return sb.toString();
  }