private static void catalogFiles(final int prefixlen, final File root, final Collection<String> files) {
        final File[] ff = root.listFiles();
        for (final File f : ff) {
            if (f.isDirectory()) {
                catalogFiles(prefixlen, f, files);
            } else {
                files.add(f.getPath().substring(prefixlen));
            }
        }
    }