/*
   * WS-Eventing
   *
   * unsubscribe: Remove a listener from events
   *
   * call-seq:
   *   client.unsubscribe(options, filter, uri, identifier) -> XmlDoc
   *
   */
  WsXmlDocH unsubscribe(client_opt_t *options, filter_t *filter, const char *resource_uri, const char *identifier) {