code: protected void addTree(SortedSet<ClassDoc> list, String heading, HtmlTree div) {
          if (!list.isEmpty()) {
              ClassDoc firstClassDoc = list.first();
              Content headingContent = getResource(heading);
              Content sectionHeading = HtmlTree.HEADING(HtmlConstants.CONTENT_HEADING, true,
                      headingContent);
              HtmlTree htmlTree;
              if (configuration.allowTag(HtmlTag.SECTION)) {
                  htmlTree = HtmlTree.SECTION(sectionHeading);
              } else {
                  div.addContent(sectionHeading);
                  htmlTree = div;
              }
              addLevelInfo(!firstClassDoc.isInterface()? firstClassDoc : null,
                      list, list == classtree.baseEnums(), htmlTree);
              if (configuration.allowTag(HtmlTag.SECTION)) {
                  div.addContent(htmlTree);
              }
          }
      }
docs: Add the heading for the tree depending upon tree type if it's a
  Class Tree or Interface tree.
  
  @param list List of classes which are at the most base level, all the
  other classes in this run will derive from these classes
  @param heading heading for the tree
  @param div the content tree to which the tree will be added