@NotNull
    private String addLineNumbers(@NotNull String source) {
        StringBuilder result = new StringBuilder((int) (source.length() * 1.2));
        int count = 1;
        for (String line : source.split("\n")) {
            result.append("<span class='lineno'>");
            result.append(String.format("%1$4d", count++));
            result.append("</span> ");
            result.append(line);
            result.append("\n");
        }
        return result.toString();
    }