code: protected Content getNavSummaryLinks() throws Exception {
          Content li = HtmlTree.LI(summaryLabel);
          li.addContent(getSpace());
          Content ulNav = HtmlTree.UL(HtmlStyle.subNavList, li);
          MemberSummaryBuilder memberSummaryBuilder = (MemberSummaryBuilder)
                  configuration.getBuilderFactory().getMemberSummaryBuilder(this);
          String[] navLinkLabels =  new String[] {
              "doclet.navNested", "doclet.navEnum", "doclet.navField", "doclet.navConstructor",
              "doclet.navMethod"
          };
          for (int i = 0; i < navLinkLabels.length; i++ ) {
              Content liNav = new HtmlTree(HtmlTag.LI);
              if (i == VisibleMemberMap.ENUM_CONSTANTS && ! classDoc.isEnum()) {
                  continue;
              }
              if (i == VisibleMemberMap.CONSTRUCTORS && classDoc.isEnum()) {
                  continue;
              }
              AbstractMemberWriter writer =
                  ((AbstractMemberWriter) memberSummaryBuilder.
                  getMemberSummaryWriter(i));
              if (writer == null) {
                  liNav.addContent(getResource(navLinkLabels[i]));
              } else {
                  writer.addNavSummaryLink(
                          memberSummaryBuilder.members(i),
                          memberSummaryBuilder.getVisibleMemberMap(i), liNav);
              }
              if (i < navLinkLabels.length-1) {
                  addNavGap(liNav);
              }
              ulNav.addContent(liNav);
          }
          return ulNav;
      }
docs: Get summary links for navigation bar.
  
  @return the content tree for the navigation summary links