Get loop index of outer loop

In jinja, the variable loop.index holds the iteration number of the current running loop.

When I have nested loops, how can I get in the inner loop the current iteration of an outer loop?

Store it in a variable, for example:

{% for i in a %}
    {% set outer_loop = loop %}
    {% for j in a %}
        {{ outer_loop.index }}
    {% endfor %}
{% endfor %}