void addZipEntry(ZipEntry entry) {
        String name = entry.getName();
        int i = name.lastIndexOf('/');
        RelativeDirectory dirname = new RelativeDirectory(name.substring(0, i+1));
        String basename = name.substring(i+1);
        if (basename.length() == 0)
            return;
        List<String> list = map.get(dirname);
        if (list == null)
            list = List.nil();
        list = list.prepend(basename);
        map.put(dirname, list);
    }