code: protected void addTree(List<ClassDoc> list, String heading, Content div) {
          if (list.size() > 0) {
              ClassDoc firstClassDoc = list.get(0);
              Content headingContent = getResource(heading);
              div.addContent(HtmlTree.HEADING(HtmlConstants.CONTENT_HEADING, true,
                      headingContent));
              addLevelInfo(!firstClassDoc.isInterface()? firstClassDoc : null,
                      list, list == classtree.baseEnums(), div);
          }
      }
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