code: protected Content getNavLinkTree() {
          Content treeLinkContent;
          PackageDoc[] packages = configuration.root.specifiedPackages();
          if (packages.length == 1 && configuration.root.specifiedClasses().length == 0) {
              treeLinkContent = getHyperLink(pathString(packages[0],
                      DocPaths.PACKAGE_TREE), treeLabel,
                      "", "");
          } else {
              treeLinkContent = getHyperLink(pathToRoot.resolve(DocPaths.OVERVIEW_TREE),
                      treeLabel, "", "");
          }
          Content li = HtmlTree.LI(treeLinkContent);
          return li;
      }
docs: Get "Tree" link in the navigation bar. If there is only one package
  specified on the command line, then the "Tree" link will be to the
  only "package-tree.html" file otherwise it will be to the
  "overview-tree.html" file.
  
  @return a content tree for the link