code: private void respondToListIdentifiers(Map<String, String> args,
                                            String baseURL,
                                            List<?> headers,
                                            ResumptionToken resumptionToken,
                                            PrintWriter out) {
          appendTop(out);
          appendRequest(args, baseURL, out);
          out.println("  <ListIdentifiers>");
          for (int i = 0; i < headers.size(); i++) {
              Header header = (Header) headers.get(i);
              appendHeader("    ", header, out);
          }
          appendResumptionToken(resumptionToken, out);
          out.println("  </ListIdentifiers>");
          appendBottom(out);
      }
docs: resumptionToken may be null