code: protected void addClassList(Content contentTree) throws IOException {
          HtmlTree ul = new HtmlTree(HtmlTag.UL);
          ul.addStyle(HtmlStyle.blockList);
          for (PackageDoc pkg : pkgSet) {
              Content markerAnchor = getMarkerAnchor(getPackageAnchorName(pkg));
              HtmlTree htmlTree = (configuration.allowTag(HtmlTag.SECTION))
                      ? HtmlTree.SECTION(markerAnchor)
                      : HtmlTree.LI(HtmlStyle.blockList, markerAnchor);
              Content link = getResource("doclet.ClassUse_Uses.of.0.in.1",
                                         getLink(new LinkInfoImpl(configuration, LinkInfoImpl.Kind.CLASS_USE_HEADER,
                                                                  classdoc)),
                                         getPackageLink(pkg, utils.getPackageName(pkg)));
              Content heading = HtmlTree.HEADING(HtmlConstants.SUMMARY_HEADING, link);
              htmlTree.addContent(heading);
              addClassUse(pkg, htmlTree);
              if (configuration.allowTag(HtmlTag.SECTION)) {
                  ul.addContent(HtmlTree.LI(HtmlStyle.blockList, htmlTree));
              } else {
                  ul.addContent(htmlTree);
              }
          }
          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