{% macro render_tree(nodes, selected=None) %}
  <ul class="category-tree">
  {% for node in nodes %}
    <li>
      {% if node.exists %}
        <a class="tree-link{% if node.name == selected %} active{% endif %}"
           href="{{ url_for('index', category=node.name) }}"{% if node.description %} title="{{ node.description }}"{% endif %}>{{ node.label }}</a>
      {% else %}
        <span class="tree-parent">{{ node.label }}</span>
      {% endif %}
      {% if node.children %}{{ render_tree(node.children, selected) }}{% endif %}
    </li>
  {% endfor %}
  </ul>
{% endmacro %}