{#  https://api.drupal.org/api/drupal/core!modules!system!templates!menu.html.twig/8
    menu_name: The machine name of the menu.
    items: A nested list of menu items. Each menu item contains:
      attributes: HTML attributes for the menu item.
      below: The menu item child items.
      title: The menu link title.
      url: The menu link url, instance of \Drupal\Core\Url
      localized_options: Menu link localized options.
#}

{# All menu and submenu items #}

<ul class="nav navbar-nav">
  {% for item in items %}
    {# Menu with submenu #}
    {% if item.below %}
      <li class="dropdown">
	<a href="{{ item.url }}" class="dropdown-toggle" data-toggle="dropdown" > {{ item.title }} <b class="caret"></b> </a>
	<ul class="dropdown-menu">
	  {# Menu -> Submenu #}
	  {% for item in item.below %}
	    <li ><a href="{{ item.url }}"> {{ item.title }} </a></li>
	  {% endfor %}
	</ul>
      </li>
    {# Menu without submenu #}
    {% else %}
      {# Menu - Link #}
      {% if item == 'node' %}
	<li class="active" ><a href="{{ item.url }}"  > {{ item.title }}</a></li>
      {% else %}
	<li ><a href="{{ item.url }}"  > {{ item.title }}</a></li>
      {% endif %}
    {% endif %}
  {% endfor %}
</ul>