/**

     * Gets a relative resource path between folder and fo.

     * @param folder root of filesystem or any other folder in folders hierarc

     * @param fo arbitrary FileObject in folder's tree

     * @return relative path between folder and fo. The returned path

     * never starts with a '/'. It never ends with a '/'. Returns <code>null</

     * if fo is not in folder's tree.

     * @see #isParentOf

     * @since 4.14

     */

    public static String getRelativePath(FileObject folder, FileObject fo) {

         if (!isParentOf(folder, fo) && folder != fo) {

            return null;

         }

        String result = fo.getPath().substring(folder.getPath().length());

        if (result.startsWith("/")) {

            result = result.substring(1);

        }

        return result;

    }