{% block block1 %}from template a.html{% endblock %}
{% block block2 %}from template a.html{% endblock %}
{% block block3 %}from template a.html{% endblock %}
{% block block4 %}
  nested block from template a.html
  {% block block5 %}
    contents of the nested block from a.html
    {% block block6 %}
      double nested block.
    {% endblock %}
  {% endblock %}
{% endblock %}