code: protected void addClassList(Content contentTree) throws IOException {
          HtmlTree ul = new HtmlTree(HtmlTag.UL);
          ul.addStyle(HtmlStyle.blockList);
          for (Iterator<PackageDoc> it = pkgSet.iterator(); it.hasNext();) {
              PackageDoc pkg = it.next();
              Content li = HtmlTree.LI(HtmlStyle.blockList, getMarkerAnchor(pkg.name()));
              Content link = getResource("doclet.ClassUse_Uses.of.0.in.1",
                      getLink(new LinkInfoImpl(configuration, LinkInfoImpl.Kind.CLASS_USE_HEADER,
                      classdoc)),
                      getPackageLink(pkg, Util.getPackageName(pkg)));
              Content heading = HtmlTree.HEADING(HtmlConstants.SUMMARY_HEADING, link);
              li.addContent(heading);
              addClassUse(pkg, li);
              ul.addContent(li);
          }
          Content li = HtmlTree.LI(HtmlStyle.blockList, ul);
          contentTree.addContent(li);
      }
docs: Add the class list that use the given class.
  
  @param contentTree the content tree to which the class list will be added