Symfony2 layout composition or inheritance - symfony

Question

I can't understand some principles about layout composition in sf2.
I have this App/Namespace/MyBundle/Resources/views/layout.html.twig template:
{% extends '::base.html.twig' %}
{% block title %}Main Title{% block subtitle %}{% endblock %}{% endblock %}
{% block head %}<h1>Placeholdertitle</h1>{% endblock %}
{% block body %}{% endblock %}
{% block sidebar %}{% endblock %}
How should I make the head.html.twig to replace the head block?
Currently I have this App/Bundle/Resources/views/head.html.twig template:
{% extends 'AppNamespaceMyBundle::layout.html.twig' %}
{% block head -%}
<h1>
Main Title
</h1>
{% endblock %}
Then I load a page with this index.html.twig:
{% extends 'AppNamespaceMyBundle::layout.html.twig' %}
{% block subtitle %} | Categories{% endblock %}
{% block body -%}
<h1>Categories list</h1>
<table class="records_list">
<thead>
<tr>
<th>Id</th>
<th>Text</th>
<th>Actions</th>
</tr>
</thead>
<tbody>
{% for entity in entities %}
<tr>
<td>{{ entity.id }}</td>
<td>{{ entity.text }}</td>
<td>
<ul>
<li>
show
</li>
<li>
edit
</li>
</ul>
</td>
</tr>
{% endfor %}
</tbody>
</table>
<ul>
<li>
<a href="{{ path('category_new') }}">
Create a new entry
</a>
</li>
</ul>
{% endblock %}
And here I get the "Placeholdertitle" instead of the "Main Title", meaning that the head.html.twig isn't used. How am I supposed to use that head.html.twig to be in the layout.html.twig?

Answer 1

From what you are explaining, I guess that you want to render Layout + Head inside your Index.
You can use the following schema:
Layout <extended by> Head <extended by> Index. This way, when you will render Index, Twig will render both Head and Layout.
Another solution is to use the use.
PS: You cannot extends both head and layout in the same index file, as Twig says: Template inheritance is one of the most powerful Twig's feature but it is limited to single inheritance;

Answer 2

Did you try to remove the "-" in your head block in head.html.twig ?