code: private void addLine(Content pre, String line, int currentLineNo) {
          if (line != null) {
              Content anchor = HtmlTree.A(configuration.htmlVersion,
                      "line." + Integer.toString(currentLineNo),
                      new StringContent(utils.replaceTabs(line)));
              pre.addContent(anchor);
              pre.addContent(NEW_LINE);
          }
      }
docs: Add a line from source to the HTML file that is generated.
  
  @param pre the content tree to which the line will be added.
  @param line the string to format.
  @param currentLineNo the current number.