{% 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 %}