protected String extraToString() {
        Map<? extends Object, ? extends Object> attr = getAttributes();
        if (attr == null || attr.size() == 0) {
            return "";
        }
        StringBuilder sb = new StringBuilder();
        for (Map.Entry<?, ?> e : attr.entrySet()) {
            sb.append(TAB_SIZE).append(e.getKey()).append(": ").append(e.getValue()).append('\n');
        }
        return sb.toString();
        
    }