<nav>
    <ul>
        {% for itemId, item in items %}
            {% if item %}
                {% set itemIsSelected = ((selectedItem is not defined and loop.first) or (selectedItem is defined and selectedItem == itemId)) -%}
                <li><a{% if itemIsSelected %} class="sel"{% endif %} href="{{ item.url }}">{{ item.label }}</a></li>
            {% endif %}
        {% endfor %}
    </ul>
</nav>