code: @Override
      public Content getNavLinkPrevious() {
          Content li;
          if (prev == null) {
              li = HtmlTree.LI(contents.prevPackageLabel);
          } else {
              DocPath path = DocPath.relativePath(packageElement, prev);
              li = HtmlTree.LI(getHyperLink(path.resolve(DocPaths.PACKAGE_SUMMARY),
                  contents.prevPackageLabel, "", ""));
          }
          return li;
      }
docs: Get "PREV PACKAGE" link in the navigation bar.
  
  @return a content tree for the previous link