code: public Content getNavLinkPrevious(DocPath prev) {
          Content li;
          if (prev != null) {
              li = HtmlTree.LI(getHyperLink(prev, contents.prevLabel, "", ""));
          }
          else
              li = HtmlTree.LI(contents.prevLabel);
          return li;
      }
docs: Get link for previous file.
  
  @param prev File name for the prev link
  @return a content tree for the link