code: public static String serializeSet(Set<?> set) {
          if (set == null) {
              set = new HashSet<String>();
          }
          XStream xstream = new XStream(new DomDriver());
          return xstream.toXML(set);
      }
docs: Serializes a List to XML. If the list is either empty or null
  the XML will indicate an empty list.
  
  @param set of Strings
  @return serialized Set