code: private void respondToListSets(Map<String, String> args,
                                     String baseURL,
                                     List<?> sets,
                                     ResumptionToken resumptionToken,
                                     PrintWriter out) {
          appendTop(out);
          appendRequest(args, baseURL, out);
          out.println("  <ListSets>");
          for (int i = 0; i < sets.size(); i++) {
              SetInfo s = (SetInfo) sets.get(i);
              out.println("    <set>");
              out.println("      <setSpec>" + s.getSpec() + "</setSpec>");
              out.println("      <setName>" + s.getName() + "</setName>");
              Iterator<String> iter = s.getDescriptions().iterator();
              while (iter.hasNext()) {
                  out.println("      <setDescription>\n");
                  out.println(iter.next());
                  out.println("      </setDescription>\n");
              }
              out.println("    </set>");
          }
          appendResumptionToken(resumptionToken, out);
          out.println("  </ListSets>");
          appendBottom(out);
      }
docs: resumptionToken may be null