frosas/1-base.html.twig

## 1-base.html.twig
<body>
    {% block body %}{% endblock %}
</body>

## 2-inner-page.html.twig
{% extends 'base.html.twig' %}

{% block body %}
    <div id="inner-page">
        {% block body %}{% endblock %}
    </div>
{% endblock %}

## 3-a-page.html.twig
{% extends 'inner-page.html.twig' %}

{% block body %}
    <p>I'm a page</p>
{% endblock %}

## 4-expected-result.html
<body>
    <div id="inner-page">
        <p>I'm a page</p>
    </div>
</body>
	{% extends 'base.html.twig' %}

	{% block body %}
	<div id="inner-page">
	{% block body %}{% endblock %}
	</div>
	{% endblock %}
	{% extends 'inner-page.html.twig' %}

	{% block body %}
	<p>I'm a page</p>
	{% endblock %}