code: protected void addLevelInfo(TypeElement parent, Collection<TypeElement> collection,
              boolean isEnum, Content contentTree) {
          if (!collection.isEmpty()) {
              Content ul = new HtmlTree(HtmlTag.UL);
              for (TypeElement local : collection) {
                  HtmlTree li = new HtmlTree(HtmlTag.LI);
                  li.addStyle(HtmlStyle.circle);
                  addPartialInfo(local, li);
                  addExtendsImplements(parent, local, li);
                  addLevelInfo(local, classtree.directSubClasses(local, isEnum),
                               isEnum, li);   // Recurse
                  ul.addContent(li);
              }
              contentTree.addContent(ul);
          }
      }
docs: Add each level of the class tree. For each sub-class or
  sub-interface indents the next level information.
  Recurses itself to add sub-classes info.
  
  @param parent the superclass or superinterface of the sset
  @param collection  a collection of the sub-classes at this level
  @param isEnum true if we are generating a tree for enums
  @param contentTree the content tree to which the level information will be added