code: public Content getNavLinkPrevious() {
          Content prevletterLabel = getResource("doclet.Prev_Letter");
          if (prev == -1) {
              return HtmlTree.LI(prevletterLabel);
          }
          else {
              Content prevLink = getHyperLink(DocPaths.indexN(prev),
                      prevletterLabel);
              return HtmlTree.LI(prevLink);
          }
      }
docs: Get link to the previous unicode character.
  
  @return a content tree for the link