is_not_a mirrors :ref:`filter-is_a`. It is particularly useful when iterating over a category and excluding members of a sub-category (iterating over all images associated with a page except images in the thumbnail category).
Example for looping over all media in a rsc but excluding the thumbnail resources:
{% for m in m.rsc[id].media|is_not_a:"thumbnail" %}
...
{% endfor %}
.. seealso:: :ref:`filter-is_a`