code: private static void addLineNo(Content pre, int lineno) {
          HtmlTree span = new HtmlTree(HtmlTag.SPAN);
          span.addStyle(HtmlStyle.sourceLineNo);
          if (lineno < 10) {
              span.addContent("00" + Integer.toString(lineno));
          } else if (lineno < 100) {
              span.addContent("0" + Integer.toString(lineno));
          } else {
              span.addContent(Integer.toString(lineno));
          }
          pre.addContent(span);
      }
docs: Add the line numbers for the source code.
  
  @param pre the content tree to which the line number will be added
  @param lineno The line number